diff --git a/.gitignore b/.gitignore
index ecbeb9a3c..bcd96f885 100644
--- a/.gitignore
+++ b/.gitignore
@@ -28,13 +28,3 @@ tests/core/pyspec/test-reports
 tests/core/pyspec/eth2spec/test_results.xml
 
 *.egg-info
-
-# flake8 config
-tox.ini
-
-# VS code files
-.vscode
-*.code-workspace
-
-# npm (for doctoc)
-package-lock.json
\ No newline at end of file
diff --git a/Makefile b/Makefile
index da87f7912..5ef25289e 100644
--- a/Makefile
+++ b/Makefile
@@ -22,6 +22,9 @@ MARKDOWN_FILES = $(wildcard $(SPEC_DIR)/phase0/*.md) $(wildcard $(SPEC_DIR)/phas
 COV_HTML_OUT=.htmlcov
 COV_INDEX_FILE=$(PY_SPEC_DIR)/$(COV_HTML_OUT)/index.html
 
+CURRENT_DIR = ${CURDIR}
+LINTER_CONFIG_FILE = $(CURRENT_DIR)/linter.ini
+
 .PHONY: clean partial_clean all test citest lint generate_tests pyspec install_test open_cov \
         install_deposit_contract_tester test_deposit_contract install_deposit_contract_compiler \
         compile_deposit_contract test_compile_deposit_contract check_toc
@@ -101,9 +104,8 @@ codespell:
 
 lint: pyspec
 	. venv/bin/activate; cd $(PY_SPEC_DIR); \
-	flake8  --ignore=E252,W504,W503 --max-line-length=120 ./eth2spec \
-	&& cd ./eth2spec && mypy --follow-imports=silent --warn-unused-ignores --ignore-missing-imports --check-untyped-defs --disallow-incomplete-defs --disallow-untyped-defs -p phase0 \
-	&& mypy --follow-imports=silent --warn-unused-ignores --ignore-missing-imports --check-untyped-defs --disallow-incomplete-defs --disallow-untyped-defs -p phase1;
+	flake8  --config $(LINTER_CONFIG_FILE) ./eth2spec \
+	&& mypy --config-file $(LINTER_CONFIG_FILE) -p eth2spec.phase0 -p eth2spec.phase1
 
 install_deposit_contract_tester:
 	cd $(DEPOSIT_CONTRACT_TESTER_DIR); python3 -m venv venv; . venv/bin/activate; pip3 install -r requirements.txt
@@ -123,8 +125,6 @@ test_compile_deposit_contract:
 	cd $(DEPOSIT_CONTRACT_COMPILER_DIR); . venv/bin/activate; \
 	python3.7 -m pytest .
 
-CURRENT_DIR = ${CURDIR}
-
 # Runs a generator, identified by param 1
 define run_generator
 	# Started!
diff --git a/linter.ini b/linter.ini
new file mode 100644
index 000000000..6575642f1
--- /dev/null
+++ b/linter.ini
@@ -0,0 +1,13 @@
+[flake8]
+ignore = E252,W504,W503
+max-line-length = 120
+
+[mypy]
+disallow_incomplete_defs = True
+disallow_untyped_defs = True
+
+warn_unused_ignores = True
+warn_unused_configs = True
+warn_redundant_casts = True
+
+ignore_missing_imports = True
diff --git a/setup.py b/setup.py
index 53f352b16..85d1a95ea 100644
--- a/setup.py
+++ b/setup.py
@@ -189,10 +189,10 @@ get_base_reward = cache_this(
     lambda state, index: (state.validators.hash_tree_root(), state.slot, index),
     _get_base_reward, lru_size=2048)
 
-_get_committee_count_at_slot = get_committee_count_at_slot
-get_committee_count_at_slot = cache_this(
+_get_committee_count_per_slot = get_committee_count_per_slot
+get_committee_count_per_slot = cache_this(
     lambda state, epoch: (state.validators.hash_tree_root(), epoch),
-    _get_committee_count_at_slot, lru_size=SLOTS_PER_EPOCH * 3)
+    _get_committee_count_per_slot, lru_size=SLOTS_PER_EPOCH * 3)
 
 _get_active_validator_indices = get_active_validator_indices
 get_active_validator_indices = cache_this(
diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index f8357be96..ec4751fc3 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -89,7 +89,7 @@
     - [`get_active_validator_indices`](#get_active_validator_indices)
     - [`get_validator_churn_limit`](#get_validator_churn_limit)
     - [`get_seed`](#get_seed)
-    - [`get_committee_count_at_slot`](#get_committee_count_at_slot)
+    - [`get_committee_count_per_slot`](#get_committee_count_per_slot)
     - [`get_beacon_committee`](#get_beacon_committee)
     - [`get_beacon_proposer_index`](#get_beacon_proposer_index)
     - [`get_total_balance`](#get_total_balance)
@@ -952,14 +952,13 @@ def get_seed(state: BeaconState, epoch: Epoch, domain_type: DomainType) -> Bytes
     return hash(domain_type + int_to_bytes(epoch, length=8) + mix)
 ```
 
-#### `get_committee_count_at_slot`
+#### `get_committee_count_per_slot`
 
 ```python
-def get_committee_count_at_slot(state: BeaconState, slot: Slot) -> uint64:
+def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
     """
-    Return the number of committees at ``slot``.
+    Return the number of committees in each slot for the given ``epoch``.
     """
-    epoch = compute_epoch_at_slot(slot)
     return max(uint64(1), min(
         MAX_COMMITTEES_PER_SLOT,
         uint64(len(get_active_validator_indices(state, epoch)) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE),
@@ -974,7 +973,7 @@ def get_beacon_committee(state: BeaconState, slot: Slot, index: CommitteeIndex)
     Return the beacon committee at ``slot`` for ``index``.
     """
     epoch = compute_epoch_at_slot(slot)
-    committees_per_slot = get_committee_count_at_slot(state, slot)
+    committees_per_slot = get_committee_count_per_slot(state, epoch)
     return compute_committee(
         indices=get_active_validator_indices(state, epoch),
         seed=get_seed(state, epoch, DOMAIN_BEACON_ATTESTER),
@@ -1724,7 +1723,7 @@ def process_attester_slashing(state: BeaconState, attester_slashing: AttesterSla
 ```python
 def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     data = attestation.data
-    assert data.index < get_committee_count_at_slot(state, data.slot)
+    assert data.index < get_committee_count_per_slot(state, data.target.epoch)
     assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
     assert data.target.epoch == compute_epoch_at_slot(data.slot)
     assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH
diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index 6c4cdd2a6..96bf856da 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -275,18 +275,22 @@ Additional global topics are used to propagate lower frequency validator message
 Attestation subnets are used to propagate unaggregated attestations to subsections of the network. Their `Name`s are:
 
 - `beacon_attestation_{subnet_id}` - These topics are used to propagate unaggregated attestations to the subnet `subnet_id` (typically beacon and persistent committees) to be aggregated before being gossiped to `beacon_aggregate_and_proof`. The following validations MUST pass before forwarding the `attestation` on the subnet.
-    - _[REJECT]_ The attestation is for the correct subnet (i.e. `compute_subnet_for_attestation(state, attestation.data.slot, attestation.data.index) == subnet_id`).
+    - _[REJECT]_ The attestation is for the correct subnet -- i.e. `compute_subnet_for_attestation(committees_per_slot, attestation.data.slot, attestation.data.index) == subnet_id`, where `committees_per_slot = get_committee_count_per_slot(state, attestation.data.target.epoch)`, which may be pre-computed along with the committee information for the signature check.
     - _[IGNORE]_ `attestation.data.slot` is within the last `ATTESTATION_PROPAGATION_SLOT_RANGE` slots (within a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance) -- i.e. `attestation.data.slot + ATTESTATION_PROPAGATION_SLOT_RANGE >= current_slot >= attestation.data.slot` (a client MAY queue future attestations for processing at the appropriate slot).
-    - _[REJECT]_ The attestation is unaggregated -- that is, it has exactly one participating validator (`len(get_attesting_indices(state, attestation.data, attestation.aggregation_bits)) == 1`).
+    - _[REJECT]_ The attestation is unaggregated -- that is, it has exactly one participating validator (`len([bit in bit attestation.aggregation_bits if bit]) == 1`, i.e. exactly 1 bit is set).
     - _[IGNORE]_ There has been no other valid attestation seen on an attestation subnet that has an identical `attestation.data.target.epoch` and participating validator index.
     - _[REJECT]_ The block being voted for (`attestation.data.beacon_block_root`) passes validation.
     - _[REJECT]_ The signature of `attestation` is valid.
 
 #### Attestations and Aggregation
 
-Attestation broadcasting is grouped into subnets defined by a topic. The number of subnets is defined via `ATTESTATION_SUBNET_COUNT`. The correct subnet for an attestation can be calculated with `compute_subnet_for_attestation`. `beacon_attestation_{subnet_id}` topics, are rotated through throughout the epoch in a similar fashion to rotating through shards in committees in Phase 1.
+Attestation broadcasting is grouped into subnets defined by a topic. The number of subnets is defined via `ATTESTATION_SUBNET_COUNT`.
+The correct subnet for an attestation can be calculated with `compute_subnet_for_attestation`.
+`beacon_attestation_{subnet_id}` topics, are rotated through throughout the epoch in a similar fashion to rotating through shards in committees in Phase 1.
+The subnets are rotated through with `committees_per_slot = get_committee_count_per_slot(state, attestation.data.target.epoch)` subnets per slot.
+
+Unaggregated attestations are sent to the subnet topic, `beacon_attestation_{compute_subnet_for_attestation(committees_per_slot, attestation.data.slot, attestation.data.index)}` as `Attestation`s.
 
-Unaggregated attestations are sent to the subnet topic, `beacon_attestation_{compute_subnet_for_attestation(state, attestation.data.slot, attestation.data.index)}` as `Attestation`s.
 
 Aggregated attestations are sent to the `beacon_aggregate_and_proof` topic as `AggregateAndProof`s.
 
diff --git a/specs/phase0/validator.md b/specs/phase0/validator.md
index b52d431d7..f234c6d29 100644
--- a/specs/phase0/validator.md
+++ b/specs/phase0/validator.md
@@ -172,8 +172,9 @@ def get_committee_assignment(state: BeaconState,
     assert epoch <= next_epoch
 
     start_slot = compute_start_slot_at_epoch(epoch)
+    committee_count_per_slot = get_committee_count_per_slot(state, epoch)
     for slot in range(start_slot, start_slot + SLOTS_PER_EPOCH):
-        for index in range(get_committee_count_at_slot(state, Slot(slot))):
+        for index in range(committee_count_per_slot):
             committee = get_beacon_committee(state, Slot(slot), CommitteeIndex(index))
             if validator_index in committee:
                 return committee, CommitteeIndex(index), Slot(slot)
@@ -199,8 +200,10 @@ The beacon chain shufflings are designed to provide a minimum of 1 epoch lookahe
 
 Specifically a validator should:
 * Call `get_committee_assignment(state, next_epoch, validator_index)` when checking for next epoch assignments.
-* Find peers of the pubsub topic `beacon_attestation_{compute_subnet_for_attestation(state, slot, committee_index)}`.
-    * If an _insufficient_ number of current peers are subscribed to the topic, the validator must discover new peers on this topic. Via the discovery protocol, find peers with an ENR containing the `attnets` entry such that `ENR["attnets"][compute_subnet_for_attestation(state, slot, committee_index)] == True`. Then validate that the peers are still persisted on the desired topic by requesting `GetMetaData` and checking the resulting `attnets` field.
+* Calculate the committees per slot for the next epoch: `committees_per_slot = get_committee_count_per_slot(state, next_epoch)`
+* Calculate the subnet index: `subnet_id = compute_subnet_for_attestation(committees_per_slot, slot, committee_index)`
+* Find peers of the pubsub topic `beacon_attestation_{subnet_id}`.
+    * If an _insufficient_ number of current peers are subscribed to the topic, the validator must discover new peers on this topic. Via the discovery protocol, find peers with an ENR containing the `attnets` entry such that `ENR["attnets"][subnet_id] == True`. Then validate that the peers are still persisted on the desired topic by requesting `GetMetaData` and checking the resulting `attnets` field.
     * If the validator is assigned to be an aggregator for the slot (see `is_aggregator()`), then subscribe to the topic.
 
 *Note*: If the validator is _not_ assigned to be an aggregator, the validator only needs sufficient number of peers on the topic to be able to publish messages. The validator does not need to _subscribe_ and listen to all messages on the topic.
@@ -425,16 +428,20 @@ def get_attestation_signature(state: BeaconState, attestation_data: AttestationD
 
 #### Broadcast attestation
 
-Finally, the validator broadcasts `attestation` to the associated attestation subnet -- the `beacon_attestation_{compute_subnet_for_attestation(state, attestation.data.slot, attestation.data.committee_index)}` pubsub topic.
+Finally, the validator broadcasts `attestation` to the associated attestation subnet, the `beacon_attestation_{subnet_id}` pubsub topic.
+
+The `subnet_id` for the `attestation` is calculated with:
+- Let `committees_per_slot = get_committee_count_per_slot(state, attestation.data.target.epoch)`.
+- Let `subnet_id = compute_subnet_for_attestation(committees_per_slot, attestation.data.slot, attestation.data.committee_index)`.
 
 ```python
-def compute_subnet_for_attestation(state: BeaconState, slot: Slot, committee_index: CommitteeIndex) -> uint64:
+def compute_subnet_for_attestation(committees_per_slot: uint64, slot: Slot, committee_index: CommitteeIndex) -> uint64:
     """
     Compute the correct subnet for an attestation for Phase 0.
     Note, this mimics expected Phase 1 behavior where attestations will be mapped to their shard subnet.
     """
     slots_since_epoch_start = slot % SLOTS_PER_EPOCH
-    committees_since_epoch_start = get_committee_count_at_slot(state, slot) * slots_since_epoch_start
+    committees_since_epoch_start = committees_per_slot * slots_since_epoch_start
 
     return uint64((committees_since_epoch_start + committee_index) % ATTESTATION_SUBNET_COUNT)
 ```
diff --git a/specs/phase1/beacon-chain.md b/specs/phase1/beacon-chain.md
index 4e4778abe..a1a940adf 100644
--- a/specs/phase1/beacon-chain.md
+++ b/specs/phase1/beacon-chain.md
@@ -54,7 +54,6 @@
     - [`get_shard_proposer_index`](#get_shard_proposer_index)
     - [`get_committee_count_delta`](#get_committee_count_delta)
     - [`get_start_shard`](#get_start_shard)
-    - [`get_shard`](#get_shard)
     - [`get_latest_slot_for_shard`](#get_latest_slot_for_shard)
     - [`get_offset_slots`](#get_offset_slots)
   - [Predicates](#predicates)
@@ -167,6 +166,8 @@ class AttestationData(Container):
     # FFG vote
     source: Checkpoint
     target: Checkpoint
+    # Shard vote
+    shard: Shard
     # Current-slot shard block root
     shard_head_root: Root
     # Shard transition root
@@ -252,7 +253,7 @@ class BeaconBlockBody(Container):
     deposits: List[Deposit, MAX_DEPOSITS]
     voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
     # Custody game
-    chunk_challenges: List[CustodyChunkResponse, MAX_CUSTODY_CHUNK_CHALLENGES]
+    chunk_challenges: List[CustodyChunkChallenge, MAX_CUSTODY_CHUNK_CHALLENGES]
     chunk_challenge_responses: List[CustodyChunkResponse, MAX_CUSTODY_CHUNK_CHALLENGE_RESPONSES]
     custody_key_reveals: List[CustodyKeyReveal, MAX_CUSTODY_KEY_REVEALS]
     early_derived_secret_reveals: List[EarlyDerivedSecretReveal, MAX_EARLY_DERIVED_SECRET_REVEALS]
@@ -493,7 +494,7 @@ def compute_offset_slots(start_slot: Slot, end_slot: Slot) -> Sequence[Slot]:
 #### `compute_updated_gasprice`
 
 ```python
-def compute_updated_gasprice(prev_gasprice: Gwei, shard_block_length: uint8) -> Gwei:
+def compute_updated_gasprice(prev_gasprice: Gwei, shard_block_length: uint64) -> Gwei:
     if shard_block_length > TARGET_SHARD_BLOCK_SIZE:
         delta = (prev_gasprice * (shard_block_length - TARGET_SHARD_BLOCK_SIZE)
                  // TARGET_SHARD_BLOCK_SIZE // GASPRICE_ADJUSTMENT_COEFFICIENT)
@@ -592,7 +593,10 @@ def get_committee_count_delta(state: BeaconState, start_slot: Slot, stop_slot: S
     """
     Return the sum of committee counts in range ``[start_slot, stop_slot)``.
     """
-    return sum(get_committee_count_at_slot(state, Slot(slot)) for slot in range(start_slot, stop_slot))
+    return sum(
+        get_committee_count_per_slot(state, compute_epoch_at_slot(Slot(slot)))
+        for slot in range(start_slot, stop_slot)
+    )
 ```
 
 #### `get_start_shard`
@@ -621,16 +625,6 @@ def get_start_shard(state: BeaconState, slot: Slot) -> Shard:
         )
 ```
 
-#### `get_shard`
-
-```python
-def get_shard(state: BeaconState, attestation: Attestation) -> Shard:
-    """
-    Return the shard that the given ``attestation`` is attesting.
-    """
-    return compute_shard_from_committee_index(state, attestation.data.index, attestation.data.slot)
-```
-
 #### `get_latest_slot_for_shard`
 
 ```python
@@ -757,7 +751,7 @@ def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
 ```python
 def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
     data = attestation.data
-    assert data.index < get_committee_count_at_slot(state, data.slot)
+    assert data.index < get_committee_count_per_slot(state, data.target.epoch)
     assert data.index < get_active_shard_count(state)
     assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
     assert data.target.epoch == compute_epoch_at_slot(data.slot)
@@ -775,6 +769,9 @@ def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
     if is_on_time_attestation(state, attestation):
         # Correct parent block root
         assert data.beacon_block_root == get_block_root_at_slot(state, compute_previous_slot(state.slot))
+        # Correct shard number
+        shard = compute_shard_from_committee_index(state, attestation.data.index, attestation.data.slot)
+        assert attestation.data.shard == shard
     # Type 2: no shard transition
     else:
         # Ensure delayed attestation
@@ -935,15 +932,18 @@ def process_crosslinks(state: BeaconState,
                        shard_transitions: Sequence[ShardTransition],
                        attestations: Sequence[Attestation]) -> None:
     on_time_attestation_slot = compute_previous_slot(state.slot)
-    committee_count = get_committee_count_at_slot(state, on_time_attestation_slot)
+    committee_count = get_committee_count_per_slot(state, compute_epoch_at_slot(on_time_attestation_slot))
     for committee_index in map(CommitteeIndex, range(committee_count)):
         # All attestations in the block for this committee/shard and current slot
+        shard = compute_shard_from_committee_index(state, committee_index, on_time_attestation_slot)
+        # Since the attestations are validated, all `shard_attestations` satisfy `attestation.data.shard == shard`
         shard_attestations = [
             attestation for attestation in attestations
             if is_on_time_attestation(state, attestation) and attestation.data.index == committee_index
         ]
-        shard = compute_shard_from_committee_index(state, committee_index, on_time_attestation_slot)
-        winning_root = process_crosslink_for_shard(state, committee_index, shard_transitions[shard], shard_attestations)
+        winning_root = process_crosslink_for_shard(
+            state, committee_index, shard_transitions[shard], shard_attestations
+        )
         if winning_root != Root():
             # Mark relevant pending attestations as creating a successful crosslink
             for pending_attestation in state.current_epoch_attestations:
diff --git a/specs/phase1/custody-game.md b/specs/phase1/custody-game.md
index 124167a35..6b767193a 100644
--- a/specs/phase1/custody-game.md
+++ b/specs/phase1/custody-game.md
@@ -302,7 +302,7 @@ def process_custody_game_operations(state: BeaconState, body: BeaconBlockBody) -
             fn(state, operation)
 
     for_ops(body.chunk_challenges, process_chunk_challenge)
-    for_ops(body.chunk_challenge_responses, process_chunk_challenge)
+    for_ops(body.chunk_challenge_responses, process_chunk_challenge_response)
     for_ops(body.custody_key_reveals, process_custody_key_reveal)
     for_ops(body.early_derived_secret_reveals, process_early_derived_secret_reveal)
     for_ops(body.custody_slashings, process_custody_slashing)
diff --git a/specs/phase1/fork-choice.md b/specs/phase1/fork-choice.md
index f92640ebb..41787cfd0 100644
--- a/specs/phase1/fork-choice.md
+++ b/specs/phase1/fork-choice.md
@@ -39,7 +39,8 @@ class LatestMessage(object):
 def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None:
     target = attestation.data.target
     beacon_block_root = attestation.data.beacon_block_root
-    shard = get_shard(store.block_states[beacon_block_root], attestation)
+    # TODO: separate shard chain vote
+    shard = attestation.data.shard
     for i in attesting_indices:
         if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
             store.latest_messages[i] = LatestMessage(
diff --git a/specs/phase1/shard-transition.md b/specs/phase1/shard-transition.md
index 24c39aa3d..50694ce68 100644
--- a/specs/phase1/shard-transition.md
+++ b/specs/phase1/shard-transition.md
@@ -70,8 +70,9 @@ def shard_state_transition(shard_state: ShardState,
     """
     shard_state.slot = block.slot
     prev_gasprice = shard_state.gasprice
-    shard_state.gasprice = compute_updated_gasprice(prev_gasprice, len(block.body))
-    if len(block.body) == 0:
+    shard_block_length = len(block.body)
+    shard_state.gasprice = compute_updated_gasprice(prev_gasprice, uint64(shard_block_length))
+    if shard_block_length == 0:
         latest_block_root = shard_state.latest_block_root
     else:
         latest_block_root = hash_tree_root(block)
@@ -123,8 +124,7 @@ def is_valid_fraud_proof(beacon_state: BeaconState,
     # 2. Check if the shard state transition result is wrong between
     # `transition.shard_states[offset_index - 1]` to `transition.shard_states[offset_index]`.
     if offset_index == 0:
-        shard = get_shard(beacon_state, attestation)
-        shard_states = beacon_parent_block.body.shard_transitions[shard].shard_states
+        shard_states = beacon_parent_block.body.shard_transitions[attestation.data.shard].shard_states
         shard_state = shard_states[len(shard_states) - 1]
     else:
         shard_state = transition.shard_states[offset_index - 1]  # Not doing the actual state updates here.
diff --git a/specs/phase1/validator.md b/specs/phase1/validator.md
index ea69f0389..f347f8757 100644
--- a/specs/phase1/validator.md
+++ b/specs/phase1/validator.md
@@ -1,4 +1,4 @@
-# Ethereum 2.0 Phase 0 -- Honest Validator
+# Ethereum 2.0 Phase 1 -- Honest Validator
 
 **Notice**: This document is a work-in-progress for researchers and implementers. This is an accompanying document to [Ethereum 2.0 Phase 1](./), which describes the expected actions of a "validator" participating in the Ethereum 2.0 Phase 1 protocol.
 
@@ -94,12 +94,14 @@ Beacon chain validator assignments to beacon committees and beacon block proposa
 Lookahead for beacon committee assignments operates in the same manner as Phase 0, but committee members must join a shard block pubsub topic in addition to the committee attestation topic.
 
 Specifically _after_ finding stable peers of attestation subnets (see Phase 0) a validator should:
-* Let `shard = compute_shard_from_committee_index(committe_index)`
+* Let `shard = compute_shard_from_committee_index(state, committee_index, slot)`
 * Subscribe to the pubsub topic `shard_{shard}_block` (attestation subnet peers should have this topic available).
 
+TODO: For now, the `state` we pass to `compute_shard_from_committee_index` is the current state without considering `len(state.shard_states)`, i.e., the result from `get_active_shard_count(state)` changes. We should fix it when we have shard count update logic.
+
 ## Beacon chain responsibilities
 
-A validator has two primary responsibilities to the beacon chain: [proposing blocks](#block-proposal) and [creating attestations](#attestations-1). Proposals happen infrequently, whereas attestations should be created once per epoch.
+A validator has two primary responsibilities to the beacon chain: [proposing blocks](#block-proposal) and [creating attestations](#attesting). Proposals happen infrequently, whereas attestations should be created once per epoch.
 
 These responsibilities are largely unchanged from Phase 0, but utilize the updated `SignedBeaconBlock`, `BeaconBlock`,  `BeaconBlockBody`, `Attestation`, and `AttestationData` definitions found in Phase 1. Below notes only the additional and modified behavior with respect to Phase 0.
 
@@ -109,7 +111,7 @@ Phase 1 adds light client committees and associated responsibilities, discussed
 
 #### Preparing for a `BeaconBlock`
 
-`slot`, `proposer_index`, `parent_root` fields are unchanged.
+`slot`, `proposer_index`, `parent_root`, `state_root` fields are unchanged.
 
 #### Constructing the `BeaconBlockBody`
 
@@ -133,10 +135,10 @@ Up to `MAX_EARLY_DERIVED_SECRET_REVEALS`, [`EarlyDerivedSecretReveal`](./custody
 
 ##### Shard transitions
 
-Exactly `MAX_SHARDS` [`ShardTransition`](./beacon-chain#shardtransition) objects are included in the block. Default each to an empty `ShardTransition()`. Then for each committee assigned to the slot with an associated `committee_index` and `shard`, set `shard_transitions[shard] = full_transitions[winning_root]` if the committee had enough weight to form a crosslink this slot.
+Exactly `MAX_SHARDS` [`ShardTransition`](./beacon-chain.md#shardtransition) objects are included in the block. Default each to an empty `ShardTransition()`. Then for each committee assigned to the slot with an associated `committee_index` and `shard`, set `shard_transitions[shard] = full_transitions[winning_root]` if the committee had enough weight to form a crosslink this slot.
 
 Specifically:
-* Call `shards, winning_roots = get_shard_winning_roots(state, block.slot, block.body.attestations)`
+* Call `shards, winning_roots = get_shard_winning_roots(state, block.body.attestations)`
 * Let `full_transitions` be a dictionary mapping from the `shard_transition_root`s found in `attestations` to the corresponding full `ShardTransition`
 * Then for each `shard` and `winning_root` in `zip(shards, winning_roots)` set `shard_transitions[shard] = full_transitions[winning_root]`
 
@@ -148,15 +150,16 @@ def get_shard_winning_roots(state: BeaconState,
     shards = []
     winning_roots = []
     online_indices = get_online_validator_indices(state)
-    committee_count = get_committee_count_at_slot(state, state.slot)
+    on_time_attestation_slot = compute_previous_slot(state.slot)
+    committee_count = get_committee_count_per_slot(state, compute_epoch_at_slot(on_time_attestation_slot))
     for committee_index in map(CommitteeIndex, range(committee_count)):
-        shard = compute_shard_from_committee_index(state, committee_index, state.slot)
+        shard = compute_shard_from_committee_index(state, committee_index, on_time_attestation_slot)
         # All attestations in the block for this committee/shard and are "on time"
         shard_attestations = [
             attestation for attestation in attestations
             if is_on_time_attestation(state, attestation) and attestation.data.index == committee_index
         ]
-        committee = get_beacon_committee(state, state.slot, committee_index)
+        committee = get_beacon_committee(state, on_time_attestation_slot, committee_index)
 
         # Loop over all shard transition roots, looking for a winning root
         shard_transition_roots = set([a.data.shard_transition_root for a in shard_attestations])
@@ -184,7 +187,7 @@ def get_shard_winning_roots(state: BeaconState,
 
 ##### Light client fields
 
-First retrieve `best_aggregate` from `get_best_light_client_aggregate` where `aggregates` is a list of valid aggregated `LightClientVote`s for the previous slot.
+First retrieve `best_aggregate` from `get_best_light_client_aggregate(block, aggregates)` where `aggregates` is a list of valid aggregated `LightClientVote`s for the previous slot.
 
 Then:
 * Set `light_client_bits = best_aggregate.aggregation_bits`
@@ -219,7 +222,7 @@ A validator is expected to create, sign, and broadcast an attestation during eac
 
 Assignments and the core of this duty are unchanged from Phase 0. There are a few additional fields related to the assigned shard chain.
 
-The `Attestation` and `AttestationData` defined in the [Phase 1 Beacon Chain spec]() utilizes `shard_transition_root: Root` rather than a full `ShardTransition`. For the purposes of the validator and p2p layer, a modified `FullAttestationData` and containing `FullAttestation` are used to send the accompanying `ShardTransition` in its entirety. Note that due to the properties of SSZ `hash_tree_root`, the root and signatures of `AttestationData` and `FullAttestationData` are equivalent.
+The `Attestation` and `AttestationData` defined in the [Phase 1 Beacon Chain spec](./beacon-chain.md) utilizes `shard_transition_root: Root` rather than a full `ShardTransition`. For the purposes of the validator and p2p layer, a modified `FullAttestationData` and containing `FullAttestation` are used to send the accompanying `ShardTransition` in its entirety. Note that due to the properties of SSZ `hash_tree_root`, the root and signatures of `AttestationData` and `FullAttestationData` are equivalent.
 
 #### `FullAttestationData`
 
diff --git a/ssz/simple-serialize.md b/ssz/simple-serialize.md
index b8a6bc9a2..700a428e8 100644
--- a/ssz/simple-serialize.md
+++ b/ssz/simple-serialize.md
@@ -66,6 +66,8 @@
 * **union**: union type containing one of the given subtypes
     * notation `Union[type_0, type_1, ...]`, e.g. `union[null, uint64]`
 
+*Note*: Both `Vector[boolean, N]` and `Bitvector[N]` are valid, yet distinct due to their different serialization requirements. Similarly, both `List[boolean, N]` and `Bitlist[N]` are valid, yet distinct. Generally `Bitvector[N]`/`Bitlist[N]` are preferred because of their serialization efficiencies.
+
 ### Variable-size and fixed-size
 
 We recursively define "variable-size" types to be lists, unions, `Bitlist` and all types that contain a variable-size type. All other types are said to be "fixed-size".
@@ -88,9 +90,9 @@ Assuming a helper function `default(type)` which returns the default value for `
 | `boolean` | `False` |
 | `Container` | `[default(type) for type in container]` |
 | `Vector[type, N]` | `[default(type)] * N` |
-| `Bitvector[boolean, N]` | `[False] * N` |
+| `Bitvector[N]` | `[False] * N` |
 | `List[type, N]` | `[]` |
-| `Bitlist[boolean, N]` | `[]` |
+| `Bitlist[N]` | `[]` |
 | `Union[type_0, type_1, ...]` | `default(type_0)` |
 
 #### `is_zero`
diff --git a/tests/core/pyspec/eth2spec/test/fork_choice/test_get_head.py b/tests/core/pyspec/eth2spec/test/fork_choice/test_get_head.py
index f9a739804..859fc797f 100644
--- a/tests/core/pyspec/eth2spec/test/fork_choice/test_get_head.py
+++ b/tests/core/pyspec/eth2spec/test/fork_choice/test_get_head.py
@@ -154,7 +154,7 @@ def test_filtered_block_tree(spec, state):
     attestations = []
     for i in range(spec.SLOTS_PER_EPOCH):
         slot = rogue_block.slot + i
-        for index in range(spec.get_committee_count_at_slot(non_viable_state, slot)):
+        for index in range(spec.get_committee_count_per_slot(non_viable_state, spec.compute_epoch_at_slot(slot))):
             attestation = get_valid_attestation(spec, non_viable_state, slot, index, signed=True)
             attestations.append(attestation)
 
diff --git a/tests/core/pyspec/eth2spec/test/fork_choice/test_on_attestation.py b/tests/core/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
index 2c50544d8..c1a5b2b33 100644
--- a/tests/core/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
@@ -26,7 +26,7 @@ def run_on_attestation(spec, state, store, attestation, valid=True):
         latest_message = spec.LatestMessage(
             epoch=attestation.data.target.epoch,
             root=attestation.data.beacon_block_root,
-            shard=spec.get_shard(state, attestation),
+            shard=attestation.data.shard,
             shard_root=attestation.data.shard_head_root,
         )
 
diff --git a/tests/core/pyspec/eth2spec/test/fork_choice/test_on_shard_head.py b/tests/core/pyspec/eth2spec/test/fork_choice/test_on_shard_head.py
index 24eeaedbe..c84f073b2 100644
--- a/tests/core/pyspec/eth2spec/test/fork_choice/test_on_shard_head.py
+++ b/tests/core/pyspec/eth2spec/test/fork_choice/test_on_shard_head.py
@@ -82,7 +82,7 @@ def apply_shard_and_beacon(spec, state, store, shard_store, shard_blocks_buffer)
             shard_transition=shard_transition,
             signed=False,
         )
-        assert spec.get_shard(state, attestation) == shard
+        assert attestation.data.shard == shard
         beacon_block.body.attestations = [attestation]
         beacon_block.body.shard_transitions = shard_transitions
 
diff --git a/tests/core/pyspec/eth2spec/test/helpers/attestations.py b/tests/core/pyspec/eth2spec/test/helpers/attestations.py
index 23eba34da..0c3f012f5 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/attestations.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/attestations.py
@@ -46,7 +46,7 @@ def run_attestation_processing(spec, state, attestation, valid=True):
     yield 'post', state
 
 
-def build_attestation_data(spec, state, slot, index, shard_transition=None, on_time=True):
+def build_attestation_data(spec, state, slot, index, shard=None, shard_transition=None, on_time=True):
     assert state.slot >= slot
 
     if slot == state.slot:
@@ -78,13 +78,15 @@ def build_attestation_data(spec, state, slot, index, shard_transition=None, on_t
     )
 
     if spec.fork == PHASE1:
+        if shard is None:
+            shard = spec.compute_shard_from_committee_index(state, attestation_data.index, attestation_data.slot)
+        attestation_data.shard = shard
+
         if shard_transition is not None:
             lastest_shard_data_root_index = len(shard_transition.shard_data_roots) - 1
             attestation_data.shard_head_root = shard_transition.shard_data_roots[lastest_shard_data_root_index]
             attestation_data.shard_transition_root = shard_transition.hash_tree_root()
         else:
-            # No shard transition -> no shard block
-            shard = spec.get_shard(state, spec.Attestation(data=attestation_data))
             if on_time:
                 shard_transition = spec.get_shard_transition(state, shard, shard_blocks=[])
                 lastest_shard_data_root_index = len(shard_transition.shard_data_roots) - 1
@@ -257,7 +259,7 @@ def next_epoch_with_attestations(spec,
         block = build_empty_block_for_next_slot(spec, post_state)
         if fill_cur_epoch and post_state.slot >= spec.MIN_ATTESTATION_INCLUSION_DELAY:
             slot_to_attest = post_state.slot - spec.MIN_ATTESTATION_INCLUSION_DELAY + 1
-            committees_per_slot = spec.get_committee_count_at_slot(state, slot_to_attest)
+            committees_per_slot = spec.get_committee_count_per_slot(state, spec.compute_epoch_at_slot(slot_to_attest))
             if slot_to_attest >= spec.compute_start_slot_at_epoch(spec.get_current_epoch(post_state)):
                 for index in range(committees_per_slot):
                     if spec.fork == PHASE1:
@@ -275,7 +277,7 @@ def next_epoch_with_attestations(spec,
 
         if fill_prev_epoch:
             slot_to_attest = post_state.slot - spec.SLOTS_PER_EPOCH + 1
-            committees_per_slot = spec.get_committee_count_at_slot(state, slot_to_attest)
+            committees_per_slot = spec.get_committee_count_per_slot(state, spec.compute_epoch_at_slot(slot_to_attest))
             for index in range(committees_per_slot):
                 prev_attestation = get_valid_attestation(
                     spec, post_state, slot_to_attest, index=index, signed=True, on_time=False)
@@ -304,7 +306,7 @@ def prepare_state_with_attestations(spec, state, participation_fn=None):
     for _ in range(spec.SLOTS_PER_EPOCH + spec.MIN_ATTESTATION_INCLUSION_DELAY):
         # create an attestation for each index in each slot in epoch
         if state.slot < next_epoch_start_slot:
-            for committee_index in range(spec.get_committee_count_at_slot(state, state.slot)):
+            for committee_index in range(spec.get_committee_count_per_slot(state, spec.get_current_epoch(state))):
                 def temp_participants_filter(comm):
                     if participation_fn is None:
                         return comm
diff --git a/tests/core/pyspec/eth2spec/test/helpers/shard_block.py b/tests/core/pyspec/eth2spec/test/helpers/shard_block.py
index ddf66f6c2..6957f2283 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/shard_block.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/shard_block.py
@@ -75,7 +75,7 @@ def get_shard_transitions(spec, parent_beacon_state, shard_blocks):
 
 def get_committee_index_of_shard(spec, state, slot, shard):  # Optional[CommitteeIndex]
     active_shard_count = spec.get_active_shard_count(state)
-    committee_count = spec.get_committee_count_at_slot(state, slot)
+    committee_count = spec.get_committee_count_per_slot(state, spec.compute_epoch_at_slot(slot))
     start_shard = spec.get_start_shard(state, slot)
     for committee_index in range(committee_count):
         if (start_shard + committee_index) % active_shard_count == shard:
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/__init__.py b/tests/core/pyspec/eth2spec/test/phase0/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase0/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/__init__.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
index 5de8913d6..28fd9ac9e 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
@@ -139,7 +139,7 @@ def test_wrong_index_for_committee_signature(spec, state):
 @spec_state_test
 @never_bls
 def test_wrong_index_for_slot(spec, state):
-    while spec.get_committee_count_at_slot(state, state.slot) >= spec.MAX_COMMITTEES_PER_SLOT:
+    while spec.get_committee_count_per_slot(state, spec.get_current_epoch(state)) >= spec.MAX_COMMITTEES_PER_SLOT:
         state.validators = state.validators[:len(state.validators) // 2]
         state.balances = state.balances[:len(state.balances) // 2]
 
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_block_header.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_block_header.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_block_header.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_block_header.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_deposit.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_deposit.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_deposit.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_deposit.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_proposer_slashing.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_proposer_slashing.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_proposer_slashing.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_proposer_slashing.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_voluntary_exit.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py
rename to tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_voluntary_exit.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/__init__.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/run_epoch_process_base.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/run_epoch_process_base.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/run_epoch_process_base.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/run_epoch_process_base.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_final_updates.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_final_updates.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_final_updates.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_final_updates.py
index 8cb09be0e..27676a59a 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_final_updates.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_final_updates.py
@@ -1,5 +1,5 @@
 from eth2spec.test.context import spec_state_test, with_all_phases
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import (
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import (
     run_epoch_processing_with, run_epoch_processing_to
 )
 from eth2spec.test.helpers.state import transition_to
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_justification_and_finalization.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_justification_and_finalization.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_justification_and_finalization.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_justification_and_finalization.py
index 09af2126d..f8504fc4f 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_justification_and_finalization.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_justification_and_finalization.py
@@ -1,5 +1,5 @@
 from eth2spec.test.context import spec_state_test, with_all_phases
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import (
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import (
     run_epoch_processing_with
 )
 from eth2spec.test.helpers.state import transition_to
@@ -27,8 +27,8 @@ def add_mock_attestations(spec, state, epoch, source, target, sufficient_support
     remaining_balance = int(total_balance * 2 // 3)  # can become negative
 
     start_slot = spec.compute_start_slot_at_epoch(epoch)
+    committees_per_slot = spec.get_committee_count_per_slot(state, epoch)
     for slot in range(start_slot, start_slot + spec.SLOTS_PER_EPOCH):
-        committees_per_slot = spec.get_committee_count_at_slot(state, slot)
         for index in range(committees_per_slot):
             # Check if we already have had sufficient balance. (and undone if we don't want it).
             # If so, do not create more attestations. (we do not have empty pending attestations normally anyway)
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_registry_updates.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_registry_updates.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_registry_updates.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_registry_updates.py
index b6597b1cf..c734010f3 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_registry_updates.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_registry_updates.py
@@ -1,7 +1,7 @@
 from eth2spec.test.helpers.deposits import mock_deposit
 from eth2spec.test.helpers.state import next_epoch, next_slots
 from eth2spec.test.context import spec_state_test, with_all_phases
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
 
 
 def run_process_registry_updates(spec, state):
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
index bafefcad6..d3744f897 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
@@ -1,4 +1,5 @@
 from eth2spec.test.context import (
+    PHASE0,
     spec_state_test, spec_test,
     with_all_phases, with_phases, single_phase,
     with_custom_state,
@@ -16,7 +17,7 @@ from eth2spec.test.helpers.attestations import (
 )
 from eth2spec.test.helpers.rewards import leaking
 from eth2spec.test.helpers.attester_slashings import get_indexed_attestation_participants
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
 from random import Random
 
 
@@ -24,7 +25,7 @@ def run_process_rewards_and_penalties(spec, state):
     yield from run_epoch_processing_with(spec, state, 'process_rewards_and_penalties')
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_genesis_epoch_no_attestations_no_penalties(spec, state):
     pre_state = state.copy()
@@ -37,7 +38,7 @@ def test_genesis_epoch_no_attestations_no_penalties(spec, state):
         assert state.balances[index] == pre_state.balances[index]
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_genesis_epoch_full_attestations_no_rewards(spec, state):
     attestations = []
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_slashings.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_slashings.py
similarity index 98%
rename from tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_slashings.py
rename to tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_slashings.py
index 23c8ce11a..5101c7548 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_slashings.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_slashings.py
@@ -1,5 +1,5 @@
 from eth2spec.test.context import spec_state_test, with_all_phases
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import (
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import (
     run_epoch_processing_with, run_epoch_processing_to
 )
 from eth2spec.test.helpers.state import next_epoch
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/rewards/__init__.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/rewards/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase0/rewards/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/rewards/test_basic.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/rewards/test_basic.py
rename to tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/rewards/test_leak.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/rewards/test_leak.py
rename to tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/rewards/test_random.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_random.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/rewards/test_random.py
rename to tests/core/pyspec/eth2spec/test/phase0/rewards/test_random.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/sanity/__init__.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/sanity/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase0/sanity/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
similarity index 99%
rename from tests/core/pyspec/eth2spec/test/phase_0/sanity/test_blocks.py
rename to tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
index 1e057e5a9..c3b1dceed 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
@@ -21,8 +21,8 @@ from eth2spec.test.helpers.deposits import prepare_state_and_deposit
 from eth2spec.test.helpers.shard_transitions import get_shard_transition_of_committee
 
 from eth2spec.test.context import (
+    PHASE0, PHASE1,
     spec_state_test, with_all_phases, expect_assertion_error, always_bls, with_phases,
-    PHASE1
 )
 
 
@@ -113,7 +113,7 @@ def process_and_sign_block_without_header_validations(spec, state, block):
     return sign_block(spec, state, block)
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_proposal_for_genesis_slot(spec, state):
     assert state.slot == spec.GENESIS_SLOT
@@ -484,7 +484,7 @@ def test_duplicate_attester_slashing(spec, state):
 
 # All AttesterSlashing tests should be adopted for Phase 1 but helper support is not yet there
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_multiple_attester_slashings_no_overlap(spec, state):
     # Skip test if config cannot handle multiple AttesterSlashings per block
@@ -525,7 +525,7 @@ def test_multiple_attester_slashings_no_overlap(spec, state):
     check_attester_slashing_effect(spec, pre_state, state, full_indices)
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_multiple_attester_slashings_partial_overlap(spec, state):
     # Skip test if config cannot handle multiple AttesterSlashings per block
@@ -740,7 +740,7 @@ def prepare_signed_exits(spec, state, indices):
 # exceeding the minimal-config randao mixes memory size.
 # Applies to all voluntary-exit sanity block tests.
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_voluntary_exit(spec, state):
     validator_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
@@ -768,7 +768,7 @@ def test_voluntary_exit(spec, state):
     assert state.validators[validator_index].exit_epoch < spec.FAR_FUTURE_EPOCH
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_double_validator_exit_same_block(spec, state):
     validator_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
@@ -789,7 +789,7 @@ def test_double_validator_exit_same_block(spec, state):
     yield 'post', None
 
 
-@with_phases(['phase0'])
+@with_phases([PHASE0])
 @spec_state_test
 def test_multiple_different_validator_exits_same_block(spec, state):
     validator_indices = [
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/sanity/test_slots.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_0/sanity/test_slots.py
rename to tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/__init__.py b/tests/core/pyspec/eth2spec/test/phase1/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase1/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/__init__.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_attestation.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_attestation.py
similarity index 91%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_attestation.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_attestation.py
index 34ff28412..a0cf7472f 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_attestation.py
@@ -1,4 +1,5 @@
 from eth2spec.test.context import (
+    PHASE0,
     with_all_phases_except,
     spec_state_test,
     always_bls,
@@ -11,7 +12,7 @@ from eth2spec.test.helpers.attestations import (
 )
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 @always_bls
 def test_on_time_success(spec, state):
@@ -22,7 +23,7 @@ def test_on_time_success(spec, state):
     yield from run_attestation_processing(spec, state, attestation)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 @always_bls
 def test_late_success(spec, state):
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_chunk_challenge.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
similarity index 96%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_chunk_challenge.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
index bdb4325fd..c9adabb62 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_chunk_challenge.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
@@ -8,11 +8,12 @@ from eth2spec.test.helpers.attestations import (
 )
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.context import (
+    PHASE0,
     with_all_phases_except,
     spec_state_test,
     expect_assertion_error,
 )
-from eth2spec.test.phase_0.block_processing.test_process_attestation import run_attestation_processing
+from eth2spec.test.phase0.block_processing.test_process_attestation import run_attestation_processing
 
 
 def run_chunk_challenge_processing(spec, state, custody_chunk_challenge, valid=True):
@@ -64,7 +65,7 @@ def run_custody_chunk_response_processing(spec, state, custody_response, valid=T
     yield 'post', state
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_challenge_appended(spec, state):
     transition_to(spec, state, state.slot + 1)
@@ -85,7 +86,7 @@ def test_challenge_appended(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_challenge_empty_element_replaced(spec, state):
     transition_to(spec, state, state.slot + 1)
@@ -108,7 +109,7 @@ def test_challenge_empty_element_replaced(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_duplicate_challenge(spec, state):
     transition_to(spec, state, state.slot + 1)
@@ -131,7 +132,7 @@ def test_duplicate_challenge(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge, valid=False)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_second_challenge(spec, state):
     transition_to(spec, state, state.slot + 1)
@@ -156,7 +157,7 @@ def test_second_challenge(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge1)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_multiple_epochs_custody(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 3)
@@ -178,7 +179,7 @@ def test_multiple_epochs_custody(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_many_epochs_custody(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 20)
@@ -200,7 +201,7 @@ def test_many_epochs_custody(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_off_chain_attestation(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH)
@@ -218,7 +219,7 @@ def test_off_chain_attestation(spec, state):
     yield from run_chunk_challenge_processing(spec, state, challenge)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH)
@@ -246,7 +247,7 @@ def test_custody_response(spec, state):
     yield from run_custody_chunk_response_processing(spec, state, custody_response)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response_multiple_epochs(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 3)
@@ -274,7 +275,7 @@ def test_custody_response_multiple_epochs(spec, state):
     yield from run_custody_chunk_response_processing(spec, state, custody_response)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response_many_epochs(spec, state):
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 20)
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_custody_key_reveal.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_key_reveal.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_custody_key_reveal.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_key_reveal.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_custody_slashing.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
similarity index 94%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_custody_slashing.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
index ec0bac82d..07cd76996 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_custody_slashing.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
@@ -9,11 +9,12 @@ from eth2spec.test.helpers.attestations import (
 from eth2spec.utils.ssz.ssz_typing import ByteList
 from eth2spec.test.helpers.state import get_balance, transition_to
 from eth2spec.test.context import (
+    PHASE0,
     with_all_phases_except,
     spec_state_test,
     expect_assertion_error,
 )
-from eth2spec.test.phase_0.block_processing.test_process_attestation import run_attestation_processing
+from eth2spec.test.phase0.block_processing.test_process_attestation import run_attestation_processing
 
 
 def run_custody_slashing_processing(spec, state, custody_slashing, valid=True, correct=True):
@@ -102,31 +103,31 @@ def run_standard_custody_slashing_test(spec,
     yield from run_custody_slashing_processing(spec, state, slashing, valid=valid, correct=correct)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_slashing(spec, state):
     yield from run_standard_custody_slashing_test(spec, state)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_incorrect_custody_slashing(spec, state):
     yield from run_standard_custody_slashing_test(spec, state, correct=False)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_multiple_epochs_custody(spec, state):
     yield from run_standard_custody_slashing_test(spec, state, shard_lateness=spec.SLOTS_PER_EPOCH * 3)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_many_epochs_custody(spec, state):
     yield from run_standard_custody_slashing_test(spec, state, shard_lateness=spec.SLOTS_PER_EPOCH * 10)
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_invalid_custody_slashing(spec, state):
     yield from run_standard_custody_slashing_test(
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_early_derived_secret_reveal.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_early_derived_secret_reveal.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_early_derived_secret_reveal.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_early_derived_secret_reveal.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_shard_transition.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/block_processing/test_process_shard_transition.py
rename to tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_challenge_deadlines.py b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
similarity index 84%
rename from tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_challenge_deadlines.py
rename to tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
index f3675732a..675b0d8da 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_challenge_deadlines.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
@@ -7,13 +7,14 @@ from eth2spec.test.helpers.attestations import (
 )
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.context import (
+    PHASE0,
     with_all_phases_except,
     spec_state_test,
 )
-from eth2spec.test.phase_0.block_processing.test_process_attestation import run_attestation_processing
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
+from eth2spec.test.phase0.block_processing.test_process_attestation import run_attestation_processing
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
 
-from eth2spec.test.phase_1.block_processing.test_process_chunk_challenge import (
+from eth2spec.test.phase1.block_processing.test_process_chunk_challenge import (
     run_chunk_challenge_processing,
 )
 
@@ -22,7 +23,7 @@ def run_process_challenge_deadlines(spec, state):
     yield from run_epoch_processing_with(spec, state, 'process_challenge_deadlines')
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_slashed_after_chunk_challenge(spec, state):
     transition_to(spec, state, state.slot + 1)
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_custody_final_updates.py b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
similarity index 91%
rename from tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_custody_final_updates.py
rename to tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
index 6ca6c8c99..630ddc3a7 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_custody_final_updates.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
@@ -1,3 +1,6 @@
+from eth2spec.test.context import (
+    PHASE0,
+)
 from eth2spec.test.helpers.custody import (
     get_valid_chunk_challenge,
     get_valid_custody_chunk_response,
@@ -12,21 +15,21 @@ from eth2spec.test.context import (
     with_all_phases_except,
     spec_state_test,
 )
-from eth2spec.test.phase_0.block_processing.test_process_attestation import run_attestation_processing
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
+from eth2spec.test.phase0.block_processing.test_process_attestation import run_attestation_processing
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
 
-from eth2spec.test.phase_1.block_processing.test_process_chunk_challenge import (
+from eth2spec.test.phase1.block_processing.test_process_chunk_challenge import (
     run_chunk_challenge_processing,
     run_custody_chunk_response_processing,
 )
-from eth2spec.test.phase_1.block_processing.test_process_custody_key_reveal import run_custody_key_reveal_processing
+from eth2spec.test.phase1.block_processing.test_process_custody_key_reveal import run_custody_key_reveal_processing
 
 
 def run_process_custody_final_updates(spec, state):
     yield from run_epoch_processing_with(spec, state, 'process_custody_final_updates')
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_delay(spec, state):
     spec.initiate_validator_exit(state, 0)
@@ -37,7 +40,7 @@ def test_validator_withdrawal_delay(spec, state):
     assert state.validators[0].withdrawable_epoch == spec.FAR_FUTURE_EPOCH
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_reenable_after_custody_reveal(spec, state):
     spec.initiate_validator_exit(state, 0)
@@ -60,7 +63,7 @@ def test_validator_withdrawal_reenable_after_custody_reveal(spec, state):
     assert state.validators[0].withdrawable_epoch < spec.FAR_FUTURE_EPOCH
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_suspend_after_chunk_challenge(spec, state):
     transition_to(spec, state, state.slot + 1)
@@ -108,7 +111,7 @@ def test_validator_withdrawal_suspend_after_chunk_challenge(spec, state):
     assert state.validators[validator_index].withdrawable_epoch == spec.FAR_FUTURE_EPOCH
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_resume_after_chunk_challenge_response(spec, state):
     transition_to(spec, state, state.slot + 1)
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_reveal_deadlines.py b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_reveal_deadlines.py
similarity index 85%
rename from tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_reveal_deadlines.py
rename to tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_reveal_deadlines.py
index 9cc0069b9..da1a60446 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/epoch_processing/test_process_reveal_deadlines.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_reveal_deadlines.py
@@ -3,18 +3,19 @@ from eth2spec.test.helpers.custody import (
 )
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.context import (
+    PHASE0,
     with_all_phases_except,
     spec_state_test,
 )
-from eth2spec.test.phase_0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
-from eth2spec.test.phase_1.block_processing.test_process_custody_key_reveal import run_custody_key_reveal_processing
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import run_epoch_processing_with
+from eth2spec.test.phase1.block_processing.test_process_custody_key_reveal import run_custody_key_reveal_processing
 
 
 def run_process_challenge_deadlines(spec, state):
     yield from run_epoch_processing_with(spec, state, 'process_challenge_deadlines')
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_slashed_after_reveal_deadline(spec, state):
     assert state.validators[0].slashed == 0
@@ -33,7 +34,7 @@ def test_validator_slashed_after_reveal_deadline(spec, state):
     assert state.validators[0].slashed == 1
 
 
-@with_all_phases_except(['phase0'])
+@with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_not_slashed_after_reveal(spec, state):
     transition_to(spec, state, spec.EPOCHS_PER_CUSTODY_PERIOD * spec.SLOTS_PER_EPOCH)
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/sanity/__init__.py b/tests/core/pyspec/eth2spec/test/phase1/sanity/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/sanity/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase1/sanity/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/sanity/test_blocks.py
rename to tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/unittests/__init__.py b/tests/core/pyspec/eth2spec/test/phase1/unittests/__init__.py
similarity index 100%
rename from tests/core/pyspec/eth2spec/test/phase_1/unittests/__init__.py
rename to tests/core/pyspec/eth2spec/test/phase1/unittests/__init__.py
diff --git a/tests/core/pyspec/eth2spec/test/phase_1/unittests/test_get_start_shard.py b/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
similarity index 80%
rename from tests/core/pyspec/eth2spec/test/phase_1/unittests/test_get_start_shard.py
rename to tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
index 27afd4a4e..c443545b9 100644
--- a/tests/core/pyspec/eth2spec/test/phase_1/unittests/test_get_start_shard.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
@@ -10,11 +10,16 @@ from eth2spec.test.helpers.state import next_epoch
 @spec_state_test
 def test_get_committee_count_delta(spec, state):
     assert spec.get_committee_count_delta(state, 0, 0) == 0
-    assert spec.get_committee_count_at_slot(state, 0) != 0
-    assert spec.get_committee_count_delta(state, 0, 1) == spec.get_committee_count_at_slot(state, 0)
-    assert spec.get_committee_count_delta(state, 1, 2) == spec.get_committee_count_at_slot(state, 1)
-    assert spec.get_committee_count_delta(state, 0, 2) == (
-        spec.get_committee_count_at_slot(state, 0) + spec.get_committee_count_at_slot(state, 1)
+    assert spec.get_committee_count_per_slot(state, 0) != 0
+    assert spec.get_committee_count_delta(state, 0, 1) == spec.get_committee_count_per_slot(state, 0)
+    assert spec.get_committee_count_delta(state, 1, 2) == spec.get_committee_count_per_slot(state, 0)
+    assert spec.get_committee_count_delta(state, 0, 2) == spec.get_committee_count_per_slot(state, 0) * 2
+    assert spec.get_committee_count_delta(state, 0, spec.SLOTS_PER_EPOCH) == (
+        spec.get_committee_count_per_slot(state, 0) * spec.SLOTS_PER_EPOCH
+    )
+    assert spec.get_committee_count_delta(state, 0, 2 * spec.SLOTS_PER_EPOCH) == (
+        spec.get_committee_count_per_slot(state, 0) * spec.SLOTS_PER_EPOCH
+        + spec.get_committee_count_per_slot(state, 1) * spec.SLOTS_PER_EPOCH
     )
 
 
diff --git a/tests/core/pyspec/eth2spec/test/test_finality.py b/tests/core/pyspec/eth2spec/test/test_finality.py
index 55ccde2d9..adbadcdf2 100644
--- a/tests/core/pyspec/eth2spec/test/test_finality.py
+++ b/tests/core/pyspec/eth2spec/test/test_finality.py
@@ -1,4 +1,4 @@
-from eth2spec.test.context import spec_state_test, never_bls, with_all_phases, with_phases
+from eth2spec.test.context import PHASE0, spec_state_test, never_bls, with_all_phases, with_phases
 from eth2spec.test.helpers.state import next_epoch_via_block
 from eth2spec.test.helpers.attestations import next_epoch_with_attestations
 
@@ -28,7 +28,7 @@ def check_finality(spec,
         assert state.finalized_checkpoint == prev_state.finalized_checkpoint
 
 
-@with_phases(["phase0"])
+@with_phases([PHASE0])
 @spec_state_test
 @never_bls
 def test_finality_no_updates_at_genesis(spec, state):
diff --git a/tests/core/pyspec/eth2spec/test/validator/test_validator_unittest.py b/tests/core/pyspec/eth2spec/test/validator/test_validator_unittest.py
index f5346e4e1..d793db789 100644
--- a/tests/core/pyspec/eth2spec/test/validator/test_validator_unittest.py
+++ b/tests/core/pyspec/eth2spec/test/validator/test_validator_unittest.py
@@ -26,7 +26,7 @@ def run_get_committee_assignment(spec, state, epoch, validator_index, valid=True
         committee, committee_index, slot = assignment
         assert spec.compute_epoch_at_slot(slot) == epoch
         assert committee == spec.get_beacon_committee(state, slot, committee_index)
-        assert committee_index < spec.get_committee_count_at_slot(state, slot)
+        assert committee_index < spec.get_committee_count_per_slot(state, epoch)
         assert validator_index in committee
         assert valid
     except AssertionError:
@@ -359,13 +359,12 @@ def test_get_attestation_signature_phase0(spec, state):
 def test_compute_subnet_for_attestation(spec, state):
     for committee_idx in range(spec.MAX_COMMITTEES_PER_SLOT):
         for slot in range(state.slot, state.slot + spec.SLOTS_PER_EPOCH):
-            actual_subnet_id = spec.compute_subnet_for_attestation(state, slot, committee_idx)
+            committees_per_slot = spec.get_committee_count_per_slot(state, spec.compute_epoch_at_slot(slot))
+            actual_subnet_id = spec.compute_subnet_for_attestation(committees_per_slot, slot, committee_idx)
 
             slots_since_epoch_start = slot % spec.SLOTS_PER_EPOCH
-            committees_since_epoch_start = spec.get_committee_count_at_slot(
-                state, slot) * slots_since_epoch_start
-            expected_subnet_id = (committees_since_epoch_start +
-                                  committee_idx) % spec.ATTESTATION_SUBNET_COUNT
+            committees_since_epoch_start = committees_per_slot * slots_since_epoch_start
+            expected_subnet_id = (committees_since_epoch_start + committee_idx) % spec.ATTESTATION_SUBNET_COUNT
 
             assert actual_subnet_id == expected_subnet_id
 
diff --git a/tests/generators/epoch_processing/main.py b/tests/generators/epoch_processing/main.py
index f3bbc21e6..13ac76f41 100644
--- a/tests/generators/epoch_processing/main.py
+++ b/tests/generators/epoch_processing/main.py
@@ -2,7 +2,7 @@ from typing import Iterable
 
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.phase1 import spec as spec_phase1
-from eth2spec.test.phase_0.epoch_processing import (
+from eth2spec.test.phase0.epoch_processing import (
     test_process_final_updates,
     test_process_justification_and_finalization,
     test_process_registry_updates,
diff --git a/tests/generators/operations/main.py b/tests/generators/operations/main.py
index 935c7aa63..6d4f6d139 100644
--- a/tests/generators/operations/main.py
+++ b/tests/generators/operations/main.py
@@ -1,6 +1,6 @@
 from typing import Iterable
 
-from eth2spec.test.phase_0.block_processing import (
+from eth2spec.test.phase0.block_processing import (
     test_process_attestation,
     test_process_attester_slashing,
     test_process_block_header,
diff --git a/tests/generators/rewards/main.py b/tests/generators/rewards/main.py
index c90943cab..5d063c434 100644
--- a/tests/generators/rewards/main.py
+++ b/tests/generators/rewards/main.py
@@ -2,7 +2,7 @@ from typing import Iterable
 
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.phase1 import spec as spec_phase1
-from eth2spec.test.phase_0.rewards import (
+from eth2spec.test.phase0.rewards import (
     test_basic,
     test_leak,
     test_random,
diff --git a/tests/generators/sanity/main.py b/tests/generators/sanity/main.py
index 89fb89838..45a1c8c4f 100644
--- a/tests/generators/sanity/main.py
+++ b/tests/generators/sanity/main.py
@@ -5,7 +5,7 @@ from gen_base import gen_runner, gen_typing
 from gen_from_tests.gen import generate_from_tests
 
 from eth2spec.test.context import PHASE0
-from eth2spec.test.phase_0.sanity import test_blocks, test_slots
+from eth2spec.test.phase0.sanity import test_blocks, test_slots
 from eth2spec.config import config_util
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.phase1 import spec as spec_phase1