diff --git a/README.md b/README.md
index d05834b9f..b74102c30 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@ The current features are:
 * [Deposit Contract](specs/phase0/deposit-contract.md)
 * [Honest Validator](specs/phase0/validator.md)
 * [P2P Networking](specs/phase0/p2p-interface.md)
+* [Weak Subjectivity](specs/phase0/weak-subjectivity.md)
 
 ### Altair
 
diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 56b6b0b79..a0aa458fc 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -28,12 +28,11 @@
   - [`Predicates`](#predicates)
     - [`eth2_fast_aggregate_verify`](#eth2_fast_aggregate_verify)
   - [Misc](#misc-2)
-    - [`get_flag_indices_and_weights`](#get_flag_indices_and_weights)
     - [`add_flag`](#add_flag)
     - [`has_flag`](#has_flag)
   - [Beacon state accessors](#beacon-state-accessors)
-    - [`get_sync_committee_indices`](#get_sync_committee_indices)
-    - [`get_sync_committee`](#get_sync_committee)
+    - [`get_next_sync_committee_indices`](#get_next_sync_committee_indices)
+    - [`get_next_sync_committee`](#get_next_sync_committee)
     - [`get_base_reward_per_increment`](#get_base_reward_per_increment)
     - [`get_base_reward`](#get_base_reward)
     - [`get_unslashed_participating_indices`](#get_unslashed_participating_indices)
@@ -99,6 +98,7 @@ Altair is the first beacon chain hard fork. Its main features are:
 | Name | Value |
 | - | - |
 | `G2_POINT_AT_INFINITY` | `BLSSignature(b'\xc0' + b'\x00' * 95)` |
+| `PARTICIPATION_FLAG_WEIGHTS` | `[TIMELY_HEAD_WEIGHT, TIMELY_SOURCE_WEIGHT, TIMELY_TARGET_WEIGHT]` |
 
 ## Configuration
 
@@ -233,20 +233,6 @@ def eth2_fast_aggregate_verify(pubkeys: Sequence[BLSPubkey], message: Bytes32, s
 
 ### Misc
 
-#### `get_flag_indices_and_weights`
-
-```python
-def get_flag_indices_and_weights() -> Sequence[Tuple[int, uint64]]:
-    """
-    Return paired tuples of participation flag indices along with associated incentivization weights.
-    """
-    return (
-        (TIMELY_HEAD_FLAG_INDEX, TIMELY_HEAD_WEIGHT),
-        (TIMELY_SOURCE_FLAG_INDEX, TIMELY_SOURCE_WEIGHT),
-        (TIMELY_TARGET_FLAG_INDEX, TIMELY_TARGET_WEIGHT),
-    )
-```
-
 #### `add_flag`
 
 ```python
@@ -271,22 +257,22 @@ def has_flag(flags: ParticipationFlags, flag_index: int) -> bool:
 
 ### Beacon state accessors
 
-#### `get_sync_committee_indices`
+#### `get_next_sync_committee_indices`
 
 ```python
-def get_sync_committee_indices(state: BeaconState, epoch: Epoch) -> Sequence[ValidatorIndex]:
+def get_next_sync_committee_indices(state: BeaconState) -> Sequence[ValidatorIndex]:
     """
     Return the sequence of sync committee indices (which may include duplicate indices)
-    for a given ``state`` and ``epoch``.
+    for the next sync committee, given a ``state`` at a sync committee period boundary.
 
-    Note: This function is not stable during a sync committee period as
-    a validator's effective balance may change enough to affect the sampling.
+    Note: Committee can contain duplicate indices for small validator sets (< SYNC_COMMITTEE_SIZE + 128)
     """
+    epoch = Epoch(get_current_epoch(state) + 1)
+
     MAX_RANDOM_BYTE = 2**8 - 1
-    base_epoch = Epoch((max(epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD, 1) - 1) * EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
-    active_validator_indices = get_active_validator_indices(state, base_epoch)
+    active_validator_indices = get_active_validator_indices(state, epoch)
     active_validator_count = uint64(len(active_validator_indices))
-    seed = get_seed(state, base_epoch, DOMAIN_SYNC_COMMITTEE)
+    seed = get_seed(state, epoch, DOMAIN_SYNC_COMMITTEE)
     i = 0
     sync_committee_indices: List[ValidatorIndex] = []
     while len(sync_committee_indices) < SYNC_COMMITTEE_SIZE:
@@ -294,31 +280,31 @@ def get_sync_committee_indices(state: BeaconState, epoch: Epoch) -> Sequence[Val
         candidate_index = active_validator_indices[shuffled_index]
         random_byte = hash(seed + uint_to_bytes(uint64(i // 32)))[i % 32]
         effective_balance = state.validators[candidate_index].effective_balance
-        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE * random_byte:  # Sample with replacement
+        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE * random_byte:
             sync_committee_indices.append(candidate_index)
         i += 1
     return sync_committee_indices
 ```
 
-#### `get_sync_committee`
+#### `get_next_sync_committee`
 
 ```python
-def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
+def get_next_sync_committee(state: BeaconState) -> SyncCommittee:
     """
-    Return the sync committee for a given ``state`` and ``epoch``.
+    Return the *next* sync committee for a given ``state``.
 
     ``SyncCommittee`` contains an aggregate pubkey that enables
     resource-constrained clients to save some computation when verifying
     the sync committee's signature.
 
-    ``SyncCommittee`` can also contain duplicate pubkeys, when ``get_sync_committee_indices``
+    ``SyncCommittee`` can also contain duplicate pubkeys, when ``get_next_sync_committee_indices``
     returns duplicate indices. Implementations must take care when handling
     optimizations relating to aggregation and verification in the presence of duplicates.
 
-    Note: This function should only be called at sync committee period boundaries, as
-    ``get_sync_committee_indices`` is not stable within a given period.
+    Note: This function should only be called at sync committee period boundaries by ``process_sync_committee_updates``
+    as ``get_next_sync_committee_indices`` is not stable within a given period.
     """
-    indices = get_sync_committee_indices(state, epoch)
+    indices = get_next_sync_committee_indices(state)
     pubkeys = [state.validators[index].pubkey for index in indices]
     aggregate_pubkey = bls.AggregatePKs(pubkeys)
     return SyncCommittee(pubkeys=pubkeys, aggregate_pubkey=aggregate_pubkey)
@@ -368,35 +354,31 @@ def get_unslashed_participating_indices(state: BeaconState, flag_index: int, epo
 #### `get_flag_index_deltas`
 
 ```python
-def get_flag_index_deltas(state: BeaconState, flag_index: int, weight: uint64) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
+def get_flag_index_deltas(state: BeaconState, flag_index: int) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
     """
     Return the deltas for a given ``flag_index`` scaled by ``weight`` by scanning through the participation flags.
     """
     rewards = [Gwei(0)] * len(state.validators)
     penalties = [Gwei(0)] * len(state.validators)
-    unslashed_participating_indices = get_unslashed_participating_indices(state, flag_index, get_previous_epoch(state))
-    increment = EFFECTIVE_BALANCE_INCREMENT  # Factored out from balances to avoid uint64 overflow
-    unslashed_participating_increments = get_total_balance(state, unslashed_participating_indices) // increment
-    active_increments = get_total_active_balance(state) // increment
+    previous_epoch = get_previous_epoch(state)
+    unslashed_participating_indices = get_unslashed_participating_indices(state, flag_index, previous_epoch)
+    weight = PARTICIPATION_FLAG_WEIGHTS[flag_index]
+    unslashed_participating_balance = get_total_balance(state, unslashed_participating_indices)
+    unslashed_participating_increments = unslashed_participating_balance // EFFECTIVE_BALANCE_INCREMENT
+    active_increments = get_total_active_balance(state) // EFFECTIVE_BALANCE_INCREMENT
     for index in get_eligible_validator_indices(state):
         base_reward = get_base_reward(state, index)
         if index in unslashed_participating_indices:
-            if is_in_inactivity_leak(state):
-                # This flag reward cancels the inactivity penalty corresponding to the flag index
-                rewards[index] += Gwei(base_reward * weight // WEIGHT_DENOMINATOR)
-            else:
+            if not is_in_inactivity_leak(state):
                 reward_numerator = base_reward * weight * unslashed_participating_increments
                 rewards[index] += Gwei(reward_numerator // (active_increments * WEIGHT_DENOMINATOR))
-        else:
+        elif flag_index != TIMELY_HEAD_FLAG_INDEX:
             penalties[index] += Gwei(base_reward * weight // WEIGHT_DENOMINATOR)
     return rewards, penalties
 ```
 
 #### Modified `get_inactivity_penalty_deltas`
 
-*Note*: The function `get_inactivity_penalty_deltas` is modified in the selection of matching target indices
-and the removal of `BASE_REWARDS_PER_EPOCH`.
-
 ```python
 def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
     """
@@ -407,11 +389,6 @@ def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], S
     previous_epoch = get_previous_epoch(state)
     matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG_INDEX, previous_epoch)
     for index in get_eligible_validator_indices(state):
-        if is_in_inactivity_leak(state):
-            # TODO: to be removed in PR 2399
-            for (_, weight) in get_flag_indices_and_weights():
-                # This inactivity penalty cancels the flag reward corresponding to the flag index
-                penalties[index] += Gwei(get_base_reward(state, index) * weight // WEIGHT_DENOMINATOR)
         if index not in matching_target_indices:
             penalty_numerator = state.validators[index].effective_balance * state.inactivity_scores[index]
             penalty_denominator = INACTIVITY_SCORE_BIAS * INACTIVITY_PENALTY_QUOTIENT_ALTAIR
@@ -505,7 +482,7 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     # Update epoch participation flags
     proposer_reward_numerator = 0
     for index in get_attesting_indices(state, data, attestation.aggregation_bits):
-        for flag_index, weight in get_flag_indices_and_weights():
+        for flag_index, weight in enumerate(PARTICIPATION_FLAG_WEIGHTS):
             if flag_index in participation_flag_indices and not has_flag(epoch_participation[index], flag_index):
                 epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
                 proposer_reward_numerator += get_base_reward(state, index) * weight
@@ -650,8 +627,7 @@ def process_rewards_and_penalties(state: BeaconState) -> None:
     if get_current_epoch(state) == GENESIS_EPOCH:
         return
 
-    flag_indices_and_numerators = get_flag_indices_and_weights()
-    flag_deltas = [get_flag_index_deltas(state, index, numerator) for (index, numerator) in flag_indices_and_numerators]
+    flag_deltas = [get_flag_index_deltas(state, flag_index) for flag_index in range(len(PARTICIPATION_FLAG_WEIGHTS))]
     deltas = flag_deltas + [get_inactivity_penalty_deltas(state)]
     for (rewards, penalties) in deltas:
         for index in range(len(state.validators)):
@@ -695,7 +671,7 @@ def process_sync_committee_updates(state: BeaconState) -> None:
     next_epoch = get_current_epoch(state) + Epoch(1)
     if next_epoch % EPOCHS_PER_SYNC_COMMITTEE_PERIOD == 0:
         state.current_sync_committee = state.next_sync_committee
-        state.next_sync_committee = get_sync_committee(state, next_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+        state.next_sync_committee = get_next_sync_committee(state)
 ```
 
 ## Initialize state for pure Altair testnets and test vectors
@@ -740,8 +716,9 @@ def initialize_beacon_state_from_eth1(eth1_block_hash: Bytes32,
     state.genesis_validators_root = hash_tree_root(state.validators)
 
     # [New in Altair] Fill in sync committees
-    state.current_sync_committee = get_sync_committee(state, get_current_epoch(state))
-    state.next_sync_committee = get_sync_committee(state, get_current_epoch(state) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+    # Note: A duplicate committee is assigned for the current and next committee at genesis
+    state.current_sync_committee = get_next_sync_committee(state)
+    state.next_sync_committee = get_next_sync_committee(state)
 
     return state
 ```
diff --git a/specs/altair/fork.md b/specs/altair/fork.md
index be562b82b..2c8b9f855 100644
--- a/specs/altair/fork.md
+++ b/specs/altair/fork.md
@@ -38,7 +38,11 @@ Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since
 
 ### Upgrading the state
 
-After `process_slots` of Phase 0 finishes, if `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH`, an irregular state change is made to upgrade to Altair.
+If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH`, an irregular state change is made to upgrade to Altair.
+
+The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `ALTAIR_FORK_EPOCH * SLOTS_PER_EPOCH`.
+Care must be taken when transitioning through the fork boundary as implementations will need a modified state transition function that deviates from the Phase 0 spec.
+In particular, the outer `state_transition` function defined in the Phase 0 spec will not expose the precise fork slot to execute the upgrade in the presence of skipped slots at the fork boundary. Instead the logic must be within `process_slots`.
 
 ```python
 def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
@@ -80,8 +84,10 @@ def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
         # Inactivity
         inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
     )
+
     # Fill in sync committees
-    post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
-    post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+    # Note: A duplicate committee is assigned for the current and next committee at the fork boundary
+    post.current_sync_committee = get_next_sync_committee(post)
+    post.next_sync_committee = get_next_sync_committee(post)
     return post
 ```
diff --git a/specs/altair/p2p-interface.md b/specs/altair/p2p-interface.md
index 85e859191..6f250b57e 100644
--- a/specs/altair/p2p-interface.md
+++ b/specs/altair/p2p-interface.md
@@ -106,9 +106,18 @@ The following validations MUST pass before forwarding the `signed_contribution_a
 
 ```python
 def get_sync_subcommittee_pubkeys(state: BeaconState, subcommittee_index: uint64) -> Sequence[BLSPubkey]:
+    # Committees assigned to `slot` sign for `slot - 1`
+    # This creates the exceptional logic below when transitioning between sync committee periods
+    next_slot_epoch = compute_epoch_at_slot(Slot(state.slot + 1))
+    if compute_sync_committee_period(get_current_epoch(state)) == compute_sync_committee_period(next_slot_epoch):
+        sync_committee = state.current_sync_committee
+    else:
+        sync_committee = state.next_sync_committee
+
+    # Return pubkeys for the subcommittee index
     sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
     i = subcommittee_index * sync_subcommittee_size
-    return state.current_sync_committee.pubkeys[i:i + sync_subcommittee_size]
+    return sync_committee.pubkeys[i:i + sync_subcommittee_size]
 ```
 
 - _[IGNORE]_ The contribution's slot is for the current slot, i.e. `contribution.slot == current_slot`.
diff --git a/specs/altair/validator.md b/specs/altair/validator.md
index dbcc73486..4aee98ad4 100644
--- a/specs/altair/validator.md
+++ b/specs/altair/validator.md
@@ -143,6 +143,11 @@ A validator determines beacon committee assignments and beacon block proposal du
 To determine sync committee assignments, a validator can run the following function: `is_assigned_to_sync_committee(state, epoch, validator_index)` where `epoch` is an epoch number within the current or next sync committee period.
 This function is a predicate indicating the presence or absence of the validator in the corresponding sync committee for the queried sync committee period.
 
+*Note*: Being assigned to a sync committee for a given `slot` means that the validator produces and broadcasts signatures for `slot - 1` for inclusion in `slot`.
+This means that when assigned to an `epoch` sync committee signatures must be produced and broadcast for slots on range `[compute_start_slot_at_epoch(epoch) - 1, compute_start_slot_at_epoch(epoch) + SLOTS_PER_EPOCH - 1)`
+rather than for the range `[compute_start_slot_at_epoch(epoch), compute_start_slot_at_epoch(epoch) + SLOTS_PER_EPOCH)`.
+To reduce complexity during the Altair fork, sync committees are not expected to produce signatures for `compute_epoch_at_slot(ALTAIR_FORK_EPOCH) - 1`.
+
 ```python
 def compute_sync_committee_period(epoch: Epoch) -> uint64:
     return epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
@@ -172,7 +177,6 @@ At any given `epoch`, the `BeaconState` contains the current `SyncCommittee` and
 Once every `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` epochs, the next `SyncCommittee` becomes the current `SyncCommittee` and the next committee is computed and stored.
 
 *Note*: The data required to compute a given committee is not cached in the `BeaconState` after committees are calculated at the period boundaries.
-This means that calling `get_sync_commitee()` in a given `epoch` can return a different result than what was computed during the relevant epoch transition.
 For this reason, *always* get committee assignments via the fields of the `BeaconState` (`current_sync_committee` and `next_sync_committee`) or use the above reference code.
 
 A validator should plan for future sync committee assignments by noting which sync committee periods they are selected for participation.
@@ -224,12 +228,12 @@ def process_sync_committee_contributions(block: BeaconBlock,
                                          contributions: Set[SyncCommitteeContribution]) -> None:
     sync_aggregate = SyncAggregate()
     signatures = []
+    sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
 
     for contribution in contributions:
         subcommittee_index = contribution.subcommittee_index
         for index, participated in enumerate(contribution.aggregation_bits):
             if participated:
-                sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
                 participant_index = sync_subcommittee_size * subcommittee_index + index
                 sync_aggregate.sync_committee_bits[participant_index] = True
         signatures.append(contribution.signature)
@@ -261,12 +265,12 @@ This process occurs each slot.
 
 ##### Prepare sync committee signature
 
-If a validator is in the current sync committee (i.e. `is_assigned_to_sync_committee()` above returns `True`), then for every slot in the current sync committee period, the validator should prepare a `SyncCommitteeSignature` according to the logic in `get_sync_committee_signature` as soon as they have determined the head block of the current slot.
+If a validator is in the current sync committee (i.e. `is_assigned_to_sync_committee()` above returns `True`), then for every `slot` in the current sync committee period, the validator should prepare a `SyncCommitteeSignature` for the previous slot (`slot - 1`) according to the logic in `get_sync_committee_signature` as soon as they have determined the head block of `slot - 1`.
 
 This logic is triggered upon the same conditions as when producing an attestation.
 Meaning, a sync committee member should produce and broadcast a `SyncCommitteeSignature` either when (a) the validator has received a valid block from the expected block proposer for the current `slot` or (b) one-third of the slot has transpired (`SECONDS_PER_SLOT / 3` seconds after the start of the slot) -- whichever comes first.
 
-`get_sync_committee_signature()` assumes `state` is the head state corresponding to processing the block up to the current slot as determined by the fork choice (including any empty slots up to the current slot processed with `process_slots` on top of the latest block), `block_root` is the root of the head block, `validator_index` is the index of the validator in the registry `state.validators` controlled by `privkey`, and `privkey` is the BLS private key for the validator.
+`get_sync_committee_signature(state, block_root, validator_index, privkey)` assumes the parameter `state` is the head state corresponding to processing the block up to the current slot as determined by the fork choice (including any empty slots up to the current slot processed with `process_slots` on top of the latest block), `block_root` is the root of the head block, `validator_index` is the index of the validator in the registry `state.validators` controlled by `privkey`, and `privkey` is the BLS private key for the validator.
 
 ```python
 def get_sync_committee_signature(state: BeaconState,
@@ -286,17 +290,20 @@ def get_sync_committee_signature(state: BeaconState,
 The validator broadcasts the assembled signature to the assigned subnet, the `sync_committee_{subnet_id}` pubsub topic.
 
 The `subnet_id` is derived from the position in the sync committee such that the sync committee is divided into "subcommittees".
-`subnet_id` can be computed via `compute_subnets_for_sync_committee()` where `state` is a `BeaconState` during the matching sync committee period.
+`subnet_id` can be computed via `compute_subnets_for_sync_committee(state, validator_index)` where `state` is a `BeaconState` during the matching sync committee period.
 
 *Note*: This function returns multiple subnets if a given validator index is included multiple times in a given sync committee across multiple subcommittees.
 
 ```python
 def compute_subnets_for_sync_committee(state: BeaconState, validator_index: ValidatorIndex) -> Sequence[uint64]:
+    next_slot_epoch = compute_epoch_at_slot(Slot(state.slot + 1))
+    if compute_sync_committee_period(get_current_epoch(state)) == compute_sync_committee_period(next_slot_epoch):
+        sync_committee = state.current_sync_committee
+    else:
+        sync_committee = state.next_sync_committee
+
     target_pubkey = state.validators[validator_index].pubkey
-    sync_committee_indices = [
-        index for index, pubkey in enumerate(state.current_sync_committee.pubkeys)
-        if pubkey == target_pubkey
-    ]
+    sync_committee_indices = [index for index, pubkey in enumerate(sync_committee.pubkeys) if pubkey == target_pubkey]
     return [
         uint64(index // (SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT))
         for index in sync_committee_indices
@@ -359,7 +366,7 @@ Set `contribution.subcommittee_index` to the index for the subcommittee index co
 ###### Aggregation bits
 
 Let `contribution.aggregation_bits` be a `Bitvector[SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT]`, where the `index`th bit is set in the `Bitvector` for each corresponding validator included in this aggregate from the corresponding subcommittee.
-An aggregator finds the index in the sync committee (as returned by `get_sync_committee_indices()`) for a given validator referenced by `sync_committee_signature.validator_index` and maps the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is set in `contribution.aggegration_bits`.
+An aggregator finds the index in the sync committee (as determined by a reverse pubkey lookup on `state.current_sync_committee.pubkeys`) for a given validator referenced by `sync_committee_signature.validator_index` and maps the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is set in `contribution.aggegration_bits`.
 
 For example, if a validator with index `2044` is pseudo-randomly sampled to sync committee index `135`. This sync committee index maps to `subcommittee_index` `1` with position `7` in the `Bitvector` for the contribution.
 
diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index e9e8e092c..297d93948 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -93,6 +93,8 @@ It consists of four main sections:
     - [Why is it called Req/Resp and not RPC?](#why-is-it-called-reqresp-and-not-rpc)
     - [Why do we allow empty responses in block requests?](#why-do-we-allow-empty-responses-in-block-requests)
     - [Why does `BeaconBlocksByRange` let the server choose which branch to send blocks from?](#why-does-beaconblocksbyrange-let-the-server-choose-which-branch-to-send-blocks-from)
+    - [Why are `BlocksByRange` requests only required to be served for the latest `MIN_EPOCHS_FOR_BLOCK_REQUESTS` epochs?](#why-are-blocksbyrange-requests-only-required-to-be-served-for-the-latest-min_epochs_for_block_requests-epochs)
+    - [Why must the proposer signature be checked when backfilling blocks in the database?](#why-must-the-proposer-signature-be-checked-when-backfilling-blocks-in-the-database)
     - [What's the effect of empty slots on the sync algorithm?](#whats-the-effect-of-empty-slots-on-the-sync-algorithm)
   - [Discovery](#discovery)
     - [Why are we using discv5 and not libp2p Kademlia DHT?](#why-are-we-using-discv5-and-not-libp2p-kademlia-dht)
@@ -171,6 +173,7 @@ This section outlines constants that are used in this spec.
 |---|---|---|
 | `GOSSIP_MAX_SIZE` | `2**20` (= 1048576, 1 MiB) | The maximum allowed size of uncompressed gossip messages. |
 | `MAX_REQUEST_BLOCKS` | `2**10` (= 1024) | Maximum number of blocks in a single request |
+| `MIN_EPOCHS_FOR_BLOCK_REQUESTS` | `MIN_VALIDATOR_WITHDRAWABILITY_DELAY + CHURN_LIMIT_QUOTIENT // 2` (= 33024, ~5 months) | The minimum epoch range over which a node must serve blocks |
 | `MAX_CHUNK_SIZE` | `2**20` (1048576, 1 MiB) | The maximum allowed size of uncompressed req/resp chunked responses. |
 | `TTFB_TIMEOUT` | `5s` | The maximum time to wait for first byte of request response (time-to-first-byte). |
 | `RESP_TIMEOUT` | `10s` | The maximum time for complete response transfer. |
@@ -179,7 +182,6 @@ This section outlines constants that are used in this spec.
 | `MESSAGE_DOMAIN_INVALID_SNAPPY` | `0x00000000` | 4-byte domain for gossip message-id isolation of *invalid* snappy messages |
 | `MESSAGE_DOMAIN_VALID_SNAPPY`  | `0x01000000` | 4-byte domain for gossip message-id isolation of *valid* snappy messages |
 
-
 ## MetaData
 
 Clients MUST locally store the following `MetaData`:
@@ -565,6 +567,8 @@ The response code can have one of the following values, encoded as a single unsi
   The response payload adheres to the `ErrorMessage` schema (described below).
 -  2: **ServerError** -- the responder encountered an error while processing the request.
   The response payload adheres to the `ErrorMessage` schema (described below).
+-  3: **ResourceUnavailable** -- the responder does not have requested resource.
+  The response payload adheres to the `ErrorMessage` schema (described below).
 
 Clients MAY use response codes above `128` to indicate alternative, erroneous request-specific responses.
 
@@ -745,10 +749,27 @@ The request MUST be encoded as an SSZ-container.
 The response MUST consist of zero or more `response_chunk`.
 Each _successful_ `response_chunk` MUST contain a single `SignedBeaconBlock` payload.
 
-Clients MUST keep a record of signed blocks seen since the start of the weak subjectivity period
-and MUST support serving requests of blocks up to their own `head_block_root`.
+Clients MUST keep a record of signed blocks seen on the epoch range
+`[max(GENESIS_EPOCH, current_epoch - MIN_EPOCHS_FOR_BLOCK_REQUESTS), current_epoch]`
+where `current_epoch` is defined by the current wall-clock time,
+and clients MUST support serving requests of blocks on this range.
 
-Clients MUST respond with at least the first block that exists in the range, if they have it, and no more than `MAX_REQUEST_BLOCKS` blocks.
+Peers that are unable to reply to block requests within the
+`MIN_EPOCHS_FOR_BLOCK_REQUESTS` epoch range MAY get descored or disconnected at any time.
+
+*Note*: The above requirement implies that nodes that start from a recent weak subjectivity checkpoint
+MUST backfill the local block database to at least epoch `current_epoch - MIN_EPOCHS_FOR_BLOCK_REQUESTS`
+to be fully compliant with `BlocksByRange` requests. To safely perform such a
+backfill of blocks to the recent state, the node MUST validate both (1) the
+proposer signatures and (2) that the blocks form a valid chain up to the most
+recent block referenced in the weak subjectivity state.
+
+*Note*: Although clients that bootstrap from a weak subjectivity checkpoint can begin
+participating in the networking immediately, other peers MAY
+disconnect and/or temporarily ban such an un-synced or semi-synced client.
+
+Clients MUST respond with at least the first block that exists in the range, if they have it,
+and no more than `MAX_REQUEST_BLOCKS` blocks.
 
 The following blocks, where they exist, MUST be sent in consecutive order.
 
@@ -1393,6 +1414,45 @@ To avoid this race condition, we allow the responding side to choose which branc
 The requesting client then goes on to validate the blocks and incorporate them in their own database
 -- because they follow the same rules, they should at this point arrive at the same canonical chain.
 
+### Why are `BlocksByRange` requests only required to be served for the latest `MIN_EPOCHS_FOR_BLOCK_REQUESTS` epochs?
+
+Due to economic finality and weak subjectivity requirements of a proof-of-stake blockchain, for a new node to safely join the network
+the node must provide a recent checkpoint found out-of-band. This checkpoint can be in the form of a `root` & `epoch` or it can be the entire
+beacon state and then a simple block sync from there to the head. We expect the latter to be the dominant UX strategy.
+
+These checkpoints *in the worst case* (i.e. very large validator set and maximal allowed safety decay) must be from the
+most recent `MIN_EPOCHS_FOR_BLOCK_REQUESTS` epochs, and thus a user must be able to block sync to the head from this starting point.
+Thus, this defines the epoch range outside which nodes may prune blocks, and
+the epoch range that a new node syncing from a checkpoint must backfill.
+
+`MIN_EPOCHS_FOR_BLOCK_REQUESTS` is calculated using the arithmetic from `compute_weak_subjectivity_period` found in the
+[weak subjectivity guide](./weak-subjectivity.md). Specifically to find this max epoch range, we use the worst case event of a very large validator size
+(`>= MIN_PER_EPOCH_CHURN_LIMIT * CHURN_LIMIT_QUOTIENT`).
+
+```python
+MIN_EPOCHS_FOR_BLOCK_REQUESTS = (
+    MIN_VALIDATOR_WITHDRAWABILITY_DELAY
+    + MAX_SAFETY_DECAY * CHURN_LIMIT_QUOTIENT // (2 * 100)
+)
+```
+
+Where `MAX_SAFETY_DECAY = 100` and thus `MIN_EPOCHS_FOR_BLOCK_REQUESTS = 33024` (~5 months).
+
+### Why must the proposer signature be checked when backfilling blocks in the database?
+
+When backfilling blocks in a database from a know safe block/state (e.g. when starting from a weak subjectivity state),
+the node not only must ensure the `BeaconBlock`s form a chain to the known safe block,
+but also must check that the proposer signature is valid in the `SignedBeaconBlock` wrapper.
+
+This is because the signature is not part of the `BeaconBlock` hash chain, and
+thus could be corrupted by an attacker serving valid `BeaconBlock`s but invalid
+signatures contained in `SignedBeaconBlock`.
+
+Although in this particular use case this does not represent a decay in safety
+(due to the assumptions of starting at a weak subjectivity checkpoint), it
+would represent invalid historic data and could be unwittingly transmitted to
+additional nodes.
+
 ### What's the effect of empty slots on the sync algorithm?
 
 When syncing one can only tell that a slot has been skipped on a particular branch
diff --git a/specs/phase0/weak-subjectivity.md b/specs/phase0/weak-subjectivity.md
index fd1b3cc28..3748a7391 100644
--- a/specs/phase0/weak-subjectivity.md
+++ b/specs/phase0/weak-subjectivity.md
@@ -136,7 +136,9 @@ A brief reference for what these values look like in practice ([reference script
 
 ## Weak Subjectivity Sync
 
-Clients should allow users to input a Weak Subjectivity Checkpoint at startup, and guarantee that any successful sync leads to the given Weak Subjectivity Checkpoint along the canonical chain. If such a sync is not possible, the client should treat this as a critical and irrecoverable failure.
+Clients should allow users to input a Weak Subjectivity Checkpoint at startup,
+and guarantee that any successful sync leads to the given Weak Subjectivity Checkpoint along the canonical chain.
+If such a sync is not possible, the client should treat this as a critical and irrecoverable failure.
 
 ### Weak Subjectivity Sync Procedure
 
diff --git a/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py b/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
index 2e5d62d11..e3ee32a93 100644
--- a/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
+++ b/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
@@ -7,7 +7,6 @@ from eth2spec.test.helpers.block_processing import run_block_processing_to
 from eth2spec.test.helpers.state import (
     state_transition_and_sign_block,
     transition_to,
-    next_epoch,
 )
 from eth2spec.test.helpers.constants import (
     MAINNET, MINIMAL,
@@ -50,9 +49,9 @@ def get_committee_indices(spec, state, duplicates=False):
     """
     state = state.copy()
     current_epoch = spec.get_current_epoch(state)
-    randao_index = current_epoch % spec.EPOCHS_PER_HISTORICAL_VECTOR
+    randao_index = (current_epoch + 1) % spec.EPOCHS_PER_HISTORICAL_VECTOR
     while True:
-        committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+        committee = spec.get_next_sync_committee_indices(state)
         if duplicates:
             if len(committee) != len(set(committee)):
                 return committee
@@ -62,23 +61,32 @@ def get_committee_indices(spec, state, duplicates=False):
         state.randao_mixes[randao_index] = hash(state.randao_mixes[randao_index])
 
 
+def compute_committee_indices(spec, state, committee):
+    """
+    Given a ``committee``, calculate and return the related indices
+    """
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee_indices = [all_pubkeys.index(pubkey) for pubkey in committee.pubkeys]
+    return committee_indices
+
+
 @with_altair_and_later
 @spec_state_test
 @always_bls
 def test_invalid_signature_missing_participant(spec, state):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    committee_indices = compute_committee_indices(spec, state, state.current_sync_committee)
     rng = random.Random(2020)
-    random_participant = rng.choice(committee)
+    random_participant = rng.choice(committee_indices)
 
     block = build_empty_block_for_next_slot(spec, state)
     # Exclude one participant whose signature was included.
     block.body.sync_aggregate = spec.SyncAggregate(
-        sync_committee_bits=[index != random_participant for index in committee],
+        sync_committee_bits=[index != random_participant for index in committee_indices],
         sync_committee_signature=compute_aggregate_sync_committee_signature(
             spec,
             state,
             block.slot - 1,
-            committee,  # full committee signs
+            committee_indices,  # full committee signs
         )
     )
     yield from run_sync_committee_processing(spec, state, block, expect_exception=True)
@@ -88,31 +96,38 @@ def test_invalid_signature_missing_participant(spec, state):
 @spec_state_test
 @always_bls
 def test_invalid_signature_extra_participant(spec, state):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    committee_indices = compute_committee_indices(spec, state, state.current_sync_committee)
     rng = random.Random(3030)
-    random_participant = rng.choice(committee)
+    random_participant = rng.choice(committee_indices)
 
     block = build_empty_block_for_next_slot(spec, state)
     # Exclude one signature even though the block claims the entire committee participated.
     block.body.sync_aggregate = spec.SyncAggregate(
-        sync_committee_bits=[True] * len(committee),
+        sync_committee_bits=[True] * len(committee_indices),
         sync_committee_signature=compute_aggregate_sync_committee_signature(
             spec,
             state,
             block.slot - 1,
-            [index for index in committee if index != random_participant],
+            [index for index in committee_indices if index != random_participant],
         )
     )
 
     yield from run_sync_committee_processing(spec, state, block, expect_exception=True)
 
 
-def compute_sync_committee_inclusion_reward(spec, state, participant_index, committee, committee_bits):
+def compute_sync_committee_inclusion_reward(spec,
+                                            state,
+                                            participant_index,
+                                            committee_indices,
+                                            committee_bits):
     total_active_increments = spec.get_total_active_balance(state) // spec.EFFECTIVE_BALANCE_INCREMENT
     total_base_rewards = spec.Gwei(spec.get_base_reward_per_increment(state) * total_active_increments)
     max_epoch_rewards = spec.Gwei(total_base_rewards * spec.SYNC_REWARD_WEIGHT // spec.WEIGHT_DENOMINATOR)
-    included_indices = [index for index, bit in zip(committee, committee_bits) if bit]
-    max_slot_rewards = spec.Gwei(max_epoch_rewards * len(included_indices) // len(committee) // spec.SLOTS_PER_EPOCH)
+    included_indices = [index for index, bit in zip(committee_indices, committee_bits) if bit]
+    max_slot_rewards = spec.Gwei(
+        max_epoch_rewards * len(included_indices)
+        // len(committee_indices) // spec.SLOTS_PER_EPOCH
+    )
 
     # Compute the participant and proposer sync rewards
     committee_effective_balance = sum([state.validators[index].effective_balance for index in included_indices])
@@ -121,23 +136,23 @@ def compute_sync_committee_inclusion_reward(spec, state, participant_index, comm
     return spec.Gwei(max_slot_rewards * effective_balance // committee_effective_balance)
 
 
-def compute_sync_committee_participant_reward(spec, state, participant_index, committee, committee_bits):
-    included_indices = [index for index, bit in zip(committee, committee_bits) if bit]
+def compute_sync_committee_participant_reward(spec, state, participant_index, committee_indices, committee_bits):
+    included_indices = [index for index, bit in zip(committee_indices, committee_bits) if bit]
     multiplicities = Counter(included_indices)
 
     inclusion_reward = compute_sync_committee_inclusion_reward(
-        spec, state, participant_index, committee, committee_bits,
+        spec, state, participant_index, committee_indices, committee_bits,
     )
     return spec.Gwei(inclusion_reward * multiplicities[participant_index])
 
 
-def compute_sync_committee_proposer_reward(spec, state, committee, committee_bits):
+def compute_sync_committee_proposer_reward(spec, state, committee_indices, committee_bits):
     proposer_reward = 0
-    for index, bit in zip(committee, committee_bits):
+    for index, bit in zip(committee_indices, committee_bits):
         if not bit:
             continue
         inclusion_reward = compute_sync_committee_inclusion_reward(
-            spec, state, index, committee, committee_bits,
+            spec, state, index, committee_indices, committee_bits,
         )
         proposer_reward_denominator = (
             (spec.WEIGHT_DENOMINATOR - spec.PROPOSER_WEIGHT)
@@ -148,15 +163,15 @@ def compute_sync_committee_proposer_reward(spec, state, committee, committee_bit
     return proposer_reward
 
 
-def validate_sync_committee_rewards(spec, pre_state, post_state, committee, committee_bits, proposer_index):
+def validate_sync_committee_rewards(spec, pre_state, post_state, committee_indices, committee_bits, proposer_index):
     for index in range(len(post_state.validators)):
         reward = 0
-        if index in committee:
+        if index in committee_indices:
             reward += compute_sync_committee_participant_reward(
                 spec,
                 pre_state,
                 index,
-                committee,
+                committee_indices,
                 committee_bits,
             )
 
@@ -164,14 +179,14 @@ def validate_sync_committee_rewards(spec, pre_state, post_state, committee, comm
             reward += compute_sync_committee_proposer_reward(
                 spec,
                 pre_state,
-                committee,
+                committee_indices,
                 committee_bits,
             )
 
         assert post_state.balances[index] == pre_state.balances[index] + reward
 
 
-def run_successful_sync_committee_test(spec, state, committee, committee_bits):
+def run_successful_sync_committee_test(spec, state, committee_indices, committee_bits):
     pre_state = state.copy()
 
     block = build_empty_block_for_next_slot(spec, state)
@@ -181,7 +196,7 @@ def run_successful_sync_committee_test(spec, state, committee, committee_bits):
             spec,
             state,
             block.slot - 1,
-            [index for index, bit in zip(committee, committee_bits) if bit],
+            [index for index, bit in zip(committee_indices, committee_bits) if bit],
         )
     )
 
@@ -191,7 +206,7 @@ def run_successful_sync_committee_test(spec, state, committee, committee_bits):
         spec,
         pre_state,
         state,
-        committee,
+        committee_indices,
         committee_bits,
         block.proposer_index,
     )
@@ -201,60 +216,60 @@ def run_successful_sync_committee_test(spec, state, committee, committee_bits):
 @with_configs([MINIMAL], reason="to create nonduplicate committee")
 @spec_state_test
 def test_sync_committee_rewards_nonduplicate_committee(spec, state):
-    committee = get_committee_indices(spec, state, duplicates=False)
-    committee_size = len(committee)
+    committee_indices = get_committee_indices(spec, state, duplicates=False)
+    committee_size = len(committee_indices)
     committee_bits = [True] * committee_size
     active_validator_count = len(spec.get_active_validator_indices(state, spec.get_current_epoch(state)))
 
     # Preconditions of this test case
     assert active_validator_count >= spec.SYNC_COMMITTEE_SIZE
-    assert committee_size == len(set(committee))
+    assert committee_size == len(set(committee_indices))
 
-    yield from run_successful_sync_committee_test(spec, state, committee, committee_bits)
+    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)
 
 
 @with_altair_and_later
 @with_configs([MAINNET], reason="to create duplicate committee")
 @spec_state_test
 def test_sync_committee_rewards_duplicate_committee(spec, state):
-    committee = get_committee_indices(spec, state, duplicates=True)
-    committee_size = len(committee)
+    committee_indices = get_committee_indices(spec, state, duplicates=True)
+    committee_size = len(committee_indices)
     committee_bits = [True] * committee_size
     active_validator_count = len(spec.get_active_validator_indices(state, spec.get_current_epoch(state)))
 
     # Preconditions of this test case
     assert active_validator_count < spec.SYNC_COMMITTEE_SIZE
-    assert committee_size > len(set(committee))
+    assert committee_size > len(set(committee_indices))
 
-    yield from run_successful_sync_committee_test(spec, state, committee, committee_bits)
+    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)
 
 
 @with_altair_and_later
 @spec_state_test
 @always_bls
 def test_sync_committee_rewards_not_full_participants(spec, state):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    committee_indices = compute_committee_indices(spec, state, state.current_sync_committee)
     rng = random.Random(1010)
-    committee_bits = [rng.choice([True, False]) for _ in committee]
+    committee_bits = [rng.choice([True, False]) for _ in committee_indices]
 
-    yield from run_successful_sync_committee_test(spec, state, committee, committee_bits)
+    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)
 
 
 @with_altair_and_later
 @spec_state_test
 @always_bls
 def test_sync_committee_rewards_empty_participants(spec, state):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
-    committee_bits = [False for _ in committee]
+    committee_indices = compute_committee_indices(spec, state, state.current_sync_committee)
+    committee_bits = [False for _ in committee_indices]
 
-    yield from run_successful_sync_committee_test(spec, state, committee, committee_bits)
+    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)
 
 
 @with_altair_and_later
 @spec_state_test
 @always_bls
 def test_invalid_signature_past_block(spec, state):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    committee_indices = compute_committee_indices(spec, state, state.current_sync_committee)
 
     blocks = []
     for _ in range(2):
@@ -262,12 +277,12 @@ def test_invalid_signature_past_block(spec, state):
         block = build_empty_block_for_next_slot(spec, state)
         # Valid sync committee signature here...
         block.body.sync_aggregate = spec.SyncAggregate(
-            sync_committee_bits=[True] * len(committee),
+            sync_committee_bits=[True] * len(committee_indices),
             sync_committee_signature=compute_aggregate_sync_committee_signature(
                 spec,
                 state,
                 block.slot - 1,
-                committee,
+                committee_indices,
             )
         )
 
@@ -277,12 +292,12 @@ def test_invalid_signature_past_block(spec, state):
     invalid_block = build_empty_block_for_next_slot(spec, state)
     # Invalid signature from a slot other than the previous
     invalid_block.body.sync_aggregate = spec.SyncAggregate(
-        sync_committee_bits=[True] * len(committee),
+        sync_committee_bits=[True] * len(committee_indices),
         sync_committee_signature=compute_aggregate_sync_committee_signature(
             spec,
             state,
             invalid_block.slot - 2,
-            committee,
+            committee_indices,
         )
     )
 
@@ -307,19 +322,18 @@ def test_invalid_signature_previous_committee(spec, state):
     transition_to(spec, state, slot_in_future_sync_committee_period)
 
     # Use the previous sync committee to produce the signature.
-    pubkeys = [validator.pubkey for validator in state.validators]
     # Ensure that the pubkey sets are different.
     assert set(old_sync_committee.pubkeys) != set(state.current_sync_committee.pubkeys)
-    committee = [pubkeys.index(pubkey) for pubkey in old_sync_committee.pubkeys]
+    committee_indices = compute_committee_indices(spec, state, old_sync_committee)
 
     block = build_empty_block_for_next_slot(spec, state)
     block.body.sync_aggregate = spec.SyncAggregate(
-        sync_committee_bits=[True] * len(committee),
+        sync_committee_bits=[True] * len(committee_indices),
         sync_committee_signature=compute_aggregate_sync_committee_signature(
             spec,
             state,
             block.slot - 1,
-            committee,
+            committee_indices,
         )
     )
 
@@ -345,15 +359,13 @@ def test_valid_signature_future_committee(spec, state):
     transition_to(spec, state, slot_in_future_sync_committee_period)
 
     sync_committee = state.current_sync_committee
+    next_sync_committee = state.next_sync_committee
 
-    expected_sync_committee = spec.get_sync_committee(state, epoch_in_future_sync_committee_period)
-
-    assert sync_committee == expected_sync_committee
+    assert next_sync_committee != sync_committee
     assert sync_committee != old_current_sync_committee
     assert sync_committee != old_next_sync_committee
 
-    pubkeys = [validator.pubkey for validator in state.validators]
-    committee_indices = [pubkeys.index(pubkey) for pubkey in sync_committee.pubkeys]
+    committee_indices = compute_committee_indices(spec, state, sync_committee)
 
     block = build_empty_block_for_next_slot(spec, state)
     block.body.sync_aggregate = spec.SyncAggregate(
@@ -367,43 +379,3 @@ def test_valid_signature_future_committee(spec, state):
     )
 
     yield from run_sync_committee_processing(spec, state, block)
-
-
-@with_altair_and_later
-@spec_state_test
-def test_sync_committee_is_only_computed_at_epoch_boundary(spec, state):
-    """
-    Sync committees can only be computed at sync committee period boundaries.
-    Ensure a client respects the committee in the state (assumed to be derived
-    in the correct way).
-    """
-    current_epoch = spec.get_current_epoch(state)
-
-    # use a "synthetic" committee to simulate the situation
-    # where ``spec.get_sync_committee`` at the sync committee
-    # period epoch boundary would have diverged some epochs into the
-    # period; ``aggregate_pubkey`` is not relevant to this test
-    pubkeys = []
-    committee_indices = []
-    i = 0
-    active_validator_count = len(spec.get_active_validator_indices(state, current_epoch))
-    while len(pubkeys) < spec.SYNC_COMMITTEE_SIZE:
-        v = state.validators[i % active_validator_count]
-        if spec.is_active_validator(v, current_epoch):
-            pubkeys.append(v.pubkey)
-            committee_indices.append(i)
-        i += 1
-
-    synthetic_committee = spec.SyncCommittee(pubkeys=pubkeys, aggregate_pubkey=spec.BLSPubkey())
-    state.current_sync_committee = synthetic_committee
-
-    assert spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD > 3
-    for _ in range(3):
-        next_epoch(spec, state)
-
-    committee = get_committee_indices(spec, state)
-    assert committee != committee_indices
-    committee_size = len(committee_indices)
-    committee_bits = [True] * committee_size
-
-    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)
diff --git a/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py b/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
index c909c791c..6af16287c 100644
--- a/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
+++ b/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
@@ -39,8 +39,7 @@ def run_sync_committees_progress_test(spec, state):
 
     # Can compute the third committee having computed final balances in the last epoch
     # of this `EPOCHS_PER_SYNC_COMMITTEE_PERIOD`
-    current_epoch = spec.get_current_epoch(state)
-    third_sync_committee = spec.get_sync_committee(state, current_epoch + 2 * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+    third_sync_committee = spec.get_next_sync_committee(state)
 
     assert state.current_sync_committee == second_sync_committee
     assert state.next_sync_committee == third_sync_committee
diff --git a/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
index ffe743531..407455a0a 100644
--- a/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
@@ -18,7 +18,8 @@ from eth2spec.test.context import (
 
 
 def run_sync_committee_sanity_test(spec, state, fraction_full=1.0):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     participants = random.sample(committee, int(len(committee) * fraction_full))
 
     yield 'pre', state
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
index 932a46ca5..554cebda8 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
@@ -46,7 +46,8 @@ def test_process_light_client_update_not_updated(spec, state):
         body_root=signed_block.message.body.hash_tree_root(),
     )
     # Sync committee signing the header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
@@ -111,7 +112,8 @@ def test_process_light_client_update_timeout(spec, state):
     )
 
     # Sync committee signing the finalized_block_header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
@@ -190,7 +192,8 @@ def test_process_light_client_update_finality_updated(spec, state):
     )
 
     # Sync committee signing the finalized_block_header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
index c8a894da6..cefaaf694 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
@@ -8,8 +8,12 @@ from eth2spec.utils import bls
 from eth2spec.utils.bls import only_with_bls
 from eth2spec.test.context import (
     with_altair_and_later,
+    with_configs,
     with_state,
 )
+from eth2spec.test.helpers.constants import (
+    MINIMAL,
+)
 
 rng = random.Random(1337)
 
@@ -91,6 +95,7 @@ def _get_sync_committee_signature(
 
 @only_with_bls()
 @with_altair_and_later
+@with_configs([MINIMAL], reason="too slow")
 @with_state
 def test_process_sync_committee_contributions(phases, spec, state):
     # skip over slots at genesis
@@ -143,20 +148,63 @@ def _subnet_for_sync_committee_index(spec, i):
     return i // (spec.SYNC_COMMITTEE_SIZE // spec.SYNC_COMMITTEE_SUBNET_COUNT)
 
 
+def _get_expected_subnets_by_pubkey(sync_committee_members):
+    expected_subnets_by_pubkey = defaultdict(list)
+    for (subnet, pubkey) in sync_committee_members:
+        expected_subnets_by_pubkey[pubkey].append(subnet)
+    return expected_subnets_by_pubkey
+
+
 @with_altair_and_later
+@with_configs([MINIMAL], reason="too slow")
 @with_state
 def test_compute_subnets_for_sync_committee(state, spec, phases):
+    # Transition to the head of the next period
+    transition_to(spec, state, spec.SLOTS_PER_EPOCH * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+
+    next_slot_epoch = spec.compute_epoch_at_slot(state.slot + 1)
+    assert (
+        spec.compute_sync_committee_period(spec.get_current_epoch(state))
+        == spec.compute_sync_committee_period(next_slot_epoch)
+    )
     some_sync_committee_members = list(
         (
             _subnet_for_sync_committee_index(spec, i),
             pubkey,
         )
+        # use current_sync_committee
         for i, pubkey in enumerate(state.current_sync_committee.pubkeys)
     )
-
-    expected_subnets_by_pubkey = defaultdict(list)
-    for (subnet, pubkey) in some_sync_committee_members:
-        expected_subnets_by_pubkey[pubkey].append(subnet)
+    expected_subnets_by_pubkey = _get_expected_subnets_by_pubkey(some_sync_committee_members)
+
+    for _, pubkey in some_sync_committee_members:
+        validator_index = _validator_index_for_pubkey(state, pubkey)
+        subnets = spec.compute_subnets_for_sync_committee(state, validator_index)
+        expected_subnets = expected_subnets_by_pubkey[pubkey]
+        assert subnets == expected_subnets
+
+
+@with_altair_and_later
+@with_configs([MINIMAL], reason="too slow")
+@with_state
+def test_compute_subnets_for_sync_committee_slot_period_boundary(state, spec, phases):
+    # Transition to the end of the period
+    transition_to(spec, state, spec.SLOTS_PER_EPOCH * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD - 1)
+
+    next_slot_epoch = spec.compute_epoch_at_slot(state.slot + 1)
+    assert (
+        spec.compute_sync_committee_period(spec.get_current_epoch(state))
+        != spec.compute_sync_committee_period(next_slot_epoch)
+    )
+    some_sync_committee_members = list(
+        (
+            _subnet_for_sync_committee_index(spec, i),
+            pubkey,
+        )
+        # use next_sync_committee
+        for i, pubkey in enumerate(state.next_sync_committee.pubkeys)
+    )
+    expected_subnets_by_pubkey = _get_expected_subnets_by_pubkey(some_sync_committee_members)
 
     for _, pubkey in some_sync_committee_members:
         validator_index = _validator_index_for_pubkey(state, pubkey)
diff --git a/tests/core/pyspec/eth2spec/test/helpers/genesis.py b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
index 4a34a5eb3..c57442766 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/genesis.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
@@ -69,9 +69,8 @@ def create_genesis_state(spec, validator_balances, activation_threshold):
 
     if spec.fork not in FORKS_BEFORE_ALTAIR:
         # Fill in sync committees
-        state.current_sync_committee = spec.get_sync_committee(state, spec.get_current_epoch(state))
-        state.next_sync_committee = (
-            spec.get_sync_committee(state, spec.get_current_epoch(state) + spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
-        )
+        # Note: A duplicate committee is assigned for the current and next committee at genesis
+        state.current_sync_committee = spec.get_next_sync_committee(state)
+        state.next_sync_committee = spec.get_next_sync_committee(state)
 
     return state
diff --git a/tests/core/pyspec/eth2spec/test/helpers/rewards.py b/tests/core/pyspec/eth2spec/test/helpers/rewards.py
index f81c1fc2a..f563c70a2 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/rewards.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/rewards.py
@@ -62,13 +62,13 @@ def run_deltas(spec, state):
 
     if is_post_altair(spec):
         def get_source_deltas(state):
-            return spec.get_flag_index_deltas(state, spec.TIMELY_SOURCE_FLAG_INDEX, spec.TIMELY_SOURCE_WEIGHT)
+            return spec.get_flag_index_deltas(state, spec.TIMELY_SOURCE_FLAG_INDEX)
 
         def get_head_deltas(state):
-            return spec.get_flag_index_deltas(state, spec.TIMELY_HEAD_FLAG_INDEX, spec.TIMELY_HEAD_WEIGHT)
+            return spec.get_flag_index_deltas(state, spec.TIMELY_HEAD_FLAG_INDEX)
 
         def get_target_deltas(state):
-            return spec.get_flag_index_deltas(state, spec.TIMELY_TARGET_FLAG_INDEX, spec.TIMELY_TARGET_WEIGHT)
+            return spec.get_flag_index_deltas(state, spec.TIMELY_TARGET_FLAG_INDEX)
 
     yield from run_attestation_component_deltas(
         spec,
@@ -133,14 +133,23 @@ def run_attestation_component_deltas(spec, state, component_delta_fn, matching_a
         validator = state.validators[index]
         enough_for_reward = has_enough_for_reward(spec, state, index)
         if index in matching_indices and not validator.slashed:
-            if enough_for_reward:
-                assert rewards[index] > 0
+            if is_post_altair(spec):
+                if not spec.is_in_inactivity_leak(state) and enough_for_reward:
+                    assert rewards[index] > 0
+                else:
+                    assert rewards[index] == 0
             else:
-                assert rewards[index] == 0
+                if enough_for_reward:
+                    assert rewards[index] > 0
+                else:
+                    assert rewards[index] == 0
+
             assert penalties[index] == 0
         else:
             assert rewards[index] == 0
-            if enough_for_reward:
+            if is_post_altair(spec) and 'head' in deltas_name:
+                assert penalties[index] == 0
+            elif enough_for_reward:
                 assert penalties[index] > 0
             else:
                 assert penalties[index] == 0
@@ -225,18 +234,19 @@ def run_get_inactivity_penalty_deltas(spec, state):
             if not is_post_altair(spec):
                 cancel_base_rewards_per_epoch = spec.BASE_REWARDS_PER_EPOCH
                 base_penalty = cancel_base_rewards_per_epoch * base_reward - spec.get_proposer_reward(state, index)
-            else:
-                base_penalty = sum(
-                    base_reward * numerator // spec.WEIGHT_DENOMINATOR
-                    for (_, numerator) in spec.get_flag_indices_and_weights()
-                )
 
             if not has_enough_for_reward(spec, state, index):
                 assert penalties[index] == 0
             elif index in matching_attesting_indices or not has_enough_for_leak_penalty(spec, state, index):
-                assert penalties[index] == base_penalty
+                if is_post_altair(spec):
+                    assert penalties[index] == 0
+                else:
+                    assert penalties[index] == base_penalty
             else:
-                assert penalties[index] > base_penalty
+                if is_post_altair(spec):
+                    assert penalties[index] > 0
+                else:
+                    assert penalties[index] > base_penalty
         else:
             assert penalties[index] == 0