diff --git a/scripts/build_spec.py b/scripts/build_spec.py
index 99c5cd69d..1f5fe1ee6 100644
--- a/scripts/build_spec.py
+++ b/scripts/build_spec.py
@@ -25,8 +25,8 @@ from eth2spec.utils.ssz.ssz_impl import (
     signing_root,
 )
 from eth2spec.utils.ssz.ssz_typing import (
-    Bit, Bool, Container, List, Vector, Bytes, uint64,
-    Bytes4, Bytes32, Bytes48, Bytes96,
+    bit, boolean, Container, List, Vector, uint64,
+    Bytes4, Bytes32, Bytes48, Bytes96, Bitlist, Bitvector,
 )
 from eth2spec.utils.bls import (
     bls_aggregate_pubkeys,
@@ -52,8 +52,8 @@ from eth2spec.utils.ssz.ssz_impl import (
     is_empty,
 )
 from eth2spec.utils.ssz.ssz_typing import (
-    Bit, Bool, Container, List, Vector, Bytes, uint64,
-    Bytes4, Bytes32, Bytes48, Bytes96,
+    bit, boolean, Container, List, Vector, Bytes, uint64,
+    Bytes4, Bytes32, Bytes48, Bytes96, Bitlist, Bitvector,
 )
 from eth2spec.utils.bls import (
     bls_aggregate_pubkeys,
@@ -174,8 +174,8 @@ def combine_constants(old_constants: Dict[str, str], new_constants: Dict[str, st
 
 
 ignored_dependencies = [
-    'Bit', 'Bool', 'Vector', 'List', 'Container', 'Hash', 'BLSPubkey', 'BLSSignature', 'Bytes', 'BytesN'
-    'Bytes4', 'Bytes32', 'Bytes48', 'Bytes96',
+    'bit', 'boolean', 'Vector', 'List', 'Container', 'Hash', 'BLSPubkey', 'BLSSignature', 'Bytes', 'BytesN'
+    'Bytes4', 'Bytes32', 'Bytes48', 'Bytes96', 'Bitlist', 'Bitvector',
     'uint8', 'uint16', 'uint32', 'uint64', 'uint128', 'uint256',
     'bytes'  # to be removed after updating spec doc
 ]
diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 70a59c41d..619bd3c01 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -82,8 +82,6 @@
         - [`bytes_to_int`](#bytes_to_int)
         - [`get_total_balance`](#get_total_balance)
         - [`get_domain`](#get_domain)
-        - [`get_bitfield_bit`](#get_bitfield_bit)
-        - [`verify_bitfield`](#verify_bitfield)
         - [`convert_to_indexed`](#convert_to_indexed)
         - [`validate_indexed_attestation`](#validate_indexed_attestation)
         - [`is_slashable_attestation_data`](#is_slashable_attestation_data)
@@ -193,6 +191,7 @@ The following values are (non-configurable) constants used throughout the specif
 | `MIN_PER_EPOCH_CHURN_LIMIT` | `2**2` (= 4) |
 | `CHURN_LIMIT_QUOTIENT` | `2**16` (= 65,536) |
 | `SHUFFLE_ROUND_COUNT` | `90` |
+| `JUSTIFICATION_BITS_LENGTH` | `4` |
 
 * For the safety of crosslinks, `TARGET_COMMITTEE_SIZE` exceeds [the recommended minimum committee size of 111](https://vitalik.ca/files/Ithaca201807_Sharding.pdf); with sufficient active validators (at least `SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE`), the shuffling algorithm ensures committee sizes of at least `TARGET_COMMITTEE_SIZE`. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)
 
@@ -307,7 +306,7 @@ class Validator(Container):
     pubkey: BLSPubkey
     withdrawal_credentials: Hash  # Commitment to pubkey for withdrawals and transfers
     effective_balance: Gwei  # Balance at stake
-    slashed: Bool
+    slashed: boolean
     # Status epochs
     activation_eligibility_epoch: Epoch  # When criteria for activation were met
     activation_epoch: Epoch
@@ -345,7 +344,7 @@ class AttestationData(Container):
 ```python
 class AttestationDataAndCustodyBit(Container):
     data: AttestationData
-    custody_bit: Bit  # Challengeable bit (SSZ-bool, 1 byte) for the custody of crosslink data
+    custody_bit: bit  # Challengeable bit (SSZ-bool, 1 byte) for the custody of crosslink data
 ```
 
 #### `IndexedAttestation`
@@ -362,7 +361,7 @@ class IndexedAttestation(Container):
 
 ```python
 class PendingAttestation(Container):
-    aggregation_bitfield: Bytes[MAX_VALIDATORS_PER_COMMITTEE // 8]
+    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
     data: AttestationData
     inclusion_delay: Slot
     proposer_index: ValidatorIndex
@@ -437,9 +436,9 @@ class AttesterSlashing(Container):
 
 ```python
 class Attestation(Container):
-    aggregation_bitfield: Bytes[MAX_VALIDATORS_PER_COMMITTEE // 8]
+    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
     data: AttestationData
-    custody_bitfield: Bytes[MAX_VALIDATORS_PER_COMMITTEE // 8]
+    custody_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
     signature: BLSSignature
 ```
 
@@ -537,7 +536,7 @@ class BeaconState(Container):
     previous_crosslinks: Vector[Crosslink, SHARD_COUNT]  # Previous epoch snapshot
     current_crosslinks: Vector[Crosslink, SHARD_COUNT]
     # Finality
-    justification_bitfield: uint64  # Bit set for every recent justified epoch
+    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]  # Bit set for every recent justified epoch
     previous_justified_checkpoint: Checkpoint  # Previous epoch snapshot
     current_justified_checkpoint: Checkpoint
     finalized_checkpoint: Checkpoint
@@ -886,13 +885,14 @@ def get_crosslink_committee(state: BeaconState, epoch: Epoch, shard: Shard) -> S
 ### `get_attesting_indices`
 
 ```python
-def get_attesting_indices(state: BeaconState, data: AttestationData, bitfield: bytes) -> Set[ValidatorIndex]:
+def get_attesting_indices(state: BeaconState,
+                          data: AttestationData,
+                          bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]) -> Set[ValidatorIndex]:
     """
     Return the set of attesting indices corresponding to ``data`` and ``bitfield``.
     """
     committee = get_crosslink_committee(state, data.target.epoch, data.crosslink.shard)
-    assert verify_bitfield(bitfield, len(committee))
-    return set(index for i, index in enumerate(committee) if get_bitfield_bit(bitfield, i) == 0b1)
+    return set(index for i, index in enumerate(committee) if bits[i])
 ```
 
 ### `int_to_bytes`
@@ -933,34 +933,6 @@ def get_domain(state: BeaconState,
     return bls_domain(domain_type, fork_version)
 ```
 
-### `get_bitfield_bit`
-
-```python
-def get_bitfield_bit(bitfield: bytes, i: int) -> int:
-    """
-    Extract the bit in ``bitfield`` at position ``i``.
-    """
-    return (bitfield[i // 8] >> (i % 8)) % 2
-```
-
-### `verify_bitfield`
-
-```python
-def verify_bitfield(bitfield: bytes, committee_size: int) -> bool:
-    """
-    Verify ``bitfield`` against the ``committee_size``.
-    """
-    if len(bitfield) != (committee_size + 7) // 8:
-        return False
-
-    # Check `bitfield` is padded with zero bits only
-    for i in range(committee_size, len(bitfield) * 8):
-        if get_bitfield_bit(bitfield, i) == 0b1:
-            return False
-
-    return True
-```
-
 ### `convert_to_indexed`
 
 ```python
@@ -968,8 +940,8 @@ def convert_to_indexed(state: BeaconState, attestation: Attestation) -> IndexedA
     """
     Convert ``attestation`` to (almost) indexed-verifiable form.
     """
-    attesting_indices = get_attesting_indices(state, attestation.data, attestation.aggregation_bitfield)
-    custody_bit_1_indices = get_attesting_indices(state, attestation.data, attestation.custody_bitfield)
+    attesting_indices = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
+    custody_bit_1_indices = get_attesting_indices(state, attestation.data, attestation.custody_bits)
     assert custody_bit_1_indices.issubset(attesting_indices)
     custody_bit_0_indices = attesting_indices.difference(custody_bit_1_indices)
 
@@ -1300,7 +1272,7 @@ def get_unslashed_attesting_indices(state: BeaconState,
                                     attestations: Sequence[PendingAttestation]) -> Set[ValidatorIndex]:
     output = set()  # type: Set[ValidatorIndex]
     for a in attestations:
-        output = output.union(get_attesting_indices(state, a.data, a.aggregation_bitfield))
+        output = output.union(get_attesting_indices(state, a.data, a.aggregation_bits))
     return set(filter(lambda index: not state.validators[index].slashed, list(output)))
 ```
 
@@ -1340,34 +1312,32 @@ def process_justification_and_finalization(state: BeaconState) -> None:
 
     # Process justifications
     state.previous_justified_checkpoint = state.current_justified_checkpoint
-    state.justification_bitfield = (state.justification_bitfield << 1) % 2**64
-    previous_epoch_matching_target_balance = get_attesting_balance(
-        state, get_matching_target_attestations(state, previous_epoch)
-    )
-    if previous_epoch_matching_target_balance * 3 >= get_total_active_balance(state) * 2:
+    state.justification_bits[1:] = state.justification_bits[:-1]
+    state.justification_bits[0] = 0b0
+    matching_target_attestations = get_matching_target_attestations(state, previous_epoch)  # Previous epoch
+    if get_attesting_balance(state, matching_target_attestations) * 3 >= get_total_active_balance(state) * 2:
         state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch,
                                                         root=get_block_root(state, previous_epoch))
-        state.justification_bitfield |= (1 << 1)
-    current_epoch_matching_target_balance = get_attesting_balance(
-        state, get_matching_target_attestations(state, current_epoch)
-    )
-    if current_epoch_matching_target_balance * 3 >= get_total_active_balance(state) * 2:
-        state.current_justified_checkpoint = Checkpoint(epoch=current_epoch, root=get_block_root(state, current_epoch))
-        state.justification_bitfield |= (1 << 0)
+        state.justification_bits[1] = 0b1
+    matching_target_attestations = get_matching_target_attestations(state, current_epoch)  # Current epoch
+    if get_attesting_balance(state, matching_target_attestations) * 3 >= get_total_active_balance(state) * 2:
+        state.current_justified_checkpoint = Checkpoint(epoch=current_epoch,
+                                                        root=get_block_root(state, current_epoch))
+        state.justification_bits[0] = 0b1
 
     # Process finalizations
-    bitfield = state.justification_bitfield
+    bits = state.justification_bits
     # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
-    if (bitfield >> 1) % 8 == 0b111 and old_previous_justified_checkpoint.epoch + 3 == current_epoch:
+    if all(bits[1:4]) and old_previous_justified_checkpoint.epoch + 3 == current_epoch:
         state.finalized_checkpoint = old_previous_justified_checkpoint
     # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
-    if (bitfield >> 1) % 4 == 0b11 and old_previous_justified_checkpoint.epoch + 2 == current_epoch:
+    if all(bits[1:3]) and old_previous_justified_checkpoint.epoch + 2 == current_epoch:
         state.finalized_checkpoint = old_previous_justified_checkpoint
     # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
-    if (bitfield >> 0) % 8 == 0b111 and old_current_justified_checkpoint.epoch + 2 == current_epoch:
+    if all(bits[0:3]) and old_current_justified_checkpoint.epoch + 2 == current_epoch:
         state.finalized_checkpoint = old_current_justified_checkpoint
     # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
-    if (bitfield >> 0) % 4 == 0b11 and old_current_justified_checkpoint.epoch + 1 == current_epoch:
+    if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch:
         state.finalized_checkpoint = old_current_justified_checkpoint
 ```
 
@@ -1423,7 +1393,7 @@ def get_attestation_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence
         index = ValidatorIndex(index)
         attestation = min([
             a for a in matching_source_attestations
-            if index in get_attesting_indices(state, a.data, a.aggregation_bitfield)
+            if index in get_attesting_indices(state, a.data, a.aggregation_bits)
         ], key=lambda a: a.inclusion_delay)
         proposer_reward = Gwei(get_base_reward(state, index) // PROPOSER_REWARD_QUOTIENT)
         rewards[attestation.proposer_index] += proposer_reward
@@ -1690,7 +1660,7 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
 
     pending_attestation = PendingAttestation(
         data=data,
-        aggregation_bitfield=attestation.aggregation_bitfield,
+        aggregation_bits=attestation.aggregation_bits,
         inclusion_delay=state.slot - attestation_slot,
         proposer_index=get_beacon_proposer_index(state),
     )
@@ -1709,7 +1679,7 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     assert data.crosslink.start_epoch == parent_crosslink.end_epoch
     assert data.crosslink.end_epoch == min(data.target.epoch, parent_crosslink.end_epoch + MAX_EPOCHS_PER_CROSSLINK)
     assert data.crosslink.data_root == ZERO_HASH  # [to be removed in phase 1]
-    
+
     # Check signature
     validate_indexed_attestation(state, convert_to_indexed(state, attestation))
 ```
diff --git a/specs/core/1_custody-game.md b/specs/core/1_custody-game.md
index 9b17b39ed..24076fef6 100644
--- a/specs/core/1_custody-game.md
+++ b/specs/core/1_custody-game.md
@@ -272,22 +272,32 @@ def get_custody_chunk_count(crosslink: Crosslink) -> int:
     return crosslink_length * chunks_per_epoch
 ```
 
+### `get_bit`
+
+```python
+def get_bit(serialization: bytes, i: int) -> int:
+    """
+    Extract the bit in ``serialization`` at position ``i``.
+    """
+    return (serialization[i // 8] >> (i % 8)) % 2
+```
+
 ### `get_custody_chunk_bit`
 
 ```python
 def get_custody_chunk_bit(key: BLSSignature, chunk: bytes) -> bool:
     # TODO: Replace with something MPC-friendly, e.g. the Legendre symbol
-    return bool(get_bitfield_bit(hash(key + chunk), 0))
+    return bool(get_bit(hash(key + chunk), 0))
 ```
 
 ### `get_chunk_bits_root`
 
 ```python
-def get_chunk_bits_root(chunk_bitfield: bytes) -> Bytes32:
+def get_chunk_bits_root(chunk_bits: bytes) -> Bytes32:
     aggregated_bits = bytearray([0] * 32)
-    for i in range(0, len(chunk_bitfield), 32):
+    for i in range(0, len(chunk_bits), 32):
         for j in range(32):
-            aggregated_bits[j] ^= chunk_bitfield[i + j]
+            aggregated_bits[j] ^= chunk_bits[i + j]
     return hash(aggregated_bits)
 ```
 
@@ -469,7 +479,7 @@ def process_chunk_challenge(state: BeaconState, challenge: CustodyChunkChallenge
     responder = state.validators[challenge.responder_index]
     assert responder.exit_epoch >= get_current_epoch(state) - MAX_CHUNK_CHALLENGE_DELAY
     # Verify the responder participated in the attestation
-    attesters = get_attesting_indices(state, challenge.attestation.data, challenge.attestation.aggregation_bitfield)
+    attesters = get_attesting_indices(state, challenge.attestation.data, challenge.attestation.aggregation_bits)
     assert challenge.responder_index in attesters
     # Verify the challenge is not a duplicate
     for record in state.custody_chunk_challenge_records:
@@ -520,8 +530,9 @@ def process_bit_challenge(state: BeaconState, challenge: CustodyBitChallenge) ->
     # Verify attestation is eligible for challenging
     responder = state.validators[challenge.responder_index]
     assert epoch + responder.max_reveal_lateness <= get_reveal_period(state, challenge.responder_index)
-    # Verify responder participated in the attestation
-    attesters = get_attesting_indices(state, attestation.data, attestation.aggregation_bitfield)
+
+    # Verify the responder participated in the attestation
+    attesters = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
     assert challenge.responder_index in attesters
     # Verifier challenger is not already challenging
     for record in state.custody_bit_challenge_records:
@@ -535,11 +546,10 @@ def process_bit_challenge(state: BeaconState, challenge: CustodyBitChallenge) ->
     assert bls_verify(responder.pubkey, hash_tree_root(epoch_to_sign), challenge.responder_key, domain)
     # Verify the chunk count
     chunk_count = get_custody_chunk_count(attestation.data.crosslink)
-    assert verify_bitfield(challenge.chunk_bits, chunk_count)
     # Verify the first bit of the hash of the chunk bits does not equal the custody bit
     committee = get_crosslink_committee(state, epoch, shard)
-    custody_bit = get_bitfield_bit(attestation.custody_bitfield, committee.index(challenge.responder_index))
-    assert custody_bit != get_bitfield_bit(get_chunk_bits_root(challenge.chunk_bits), 0)
+    custody_bit = attestation.custody_bits[committee.index(challenge.responder_index)]
+    assert custody_bit != get_bit(get_chunk_bits_root(challenge.chunk_bits), 0)
     # Add new bit challenge record
     new_record = CustodyBitChallengeRecord(
         challenge_index=state.custody_challenge_index,
@@ -631,7 +641,7 @@ def process_bit_challenge_response(state: BeaconState,
     )
     # Verify the chunk bit does not match the challenge chunk bit
     assert (get_custody_chunk_bit(challenge.responder_key, response.chunk)
-            != get_bitfield_bit(challenge.chunk_bits_leaf, response.chunk_index % 256))
+            != get_bit(challenge.chunk_bits_leaf, response.chunk_index % 256))
     # Clear the challenge
     records = state.custody_bit_challenge_records
     records[records.index(challenge)] = CustodyBitChallengeRecord()
diff --git a/specs/core/1_shard-data-chains.md b/specs/core/1_shard-data-chains.md
index d1c86eca2..432820b7c 100644
--- a/specs/core/1_shard-data-chains.md
+++ b/specs/core/1_shard-data-chains.md
@@ -92,7 +92,7 @@ class ShardAttestation(Container):
         slot: Slot
         shard: Shard
         shard_block_root: Bytes32
-    aggregation_bitfield: Bytes[PLACEHOLDER]
+    aggregation_bits: Bitlist[PLACEHOLDER]
     aggregate_signature: BLSSignature
 ```
 
@@ -230,10 +230,9 @@ def verify_shard_attestation_signature(state: BeaconState,
                                        attestation: ShardAttestation) -> None:
     data = attestation.data
     persistent_committee = get_persistent_committee(state, data.shard, data.slot)
-    assert verify_bitfield(attestation.aggregation_bitfield, len(persistent_committee))
     pubkeys = []
     for i, index in enumerate(persistent_committee):
-        if get_bitfield_bit(attestation.aggregation_bitfield, i) == 0b1:
+        if attestation.aggregation_bits[i]:
             validator = state.validators[index]
             assert is_active_validator(validator, get_current_epoch(state))
             pubkeys.append(validator.pubkey)
diff --git a/specs/light_client/sync_protocol.md b/specs/light_client/sync_protocol.md
index cb7b6ce10..9a089720b 100644
--- a/specs/light_client/sync_protocol.md
+++ b/specs/light_client/sync_protocol.md
@@ -168,7 +168,7 @@ If a client wants to update its `finalized_header` it asks the network for a `Bl
 {
     'header': BeaconBlockHeader,
     'shard_aggregate_signature': BLSSignature,
-    'shard_bitfield': 'bytes',
+    'shard_bits': Bitlist[PLACEHOLDER],
     'shard_parent_block': ShardBlock,
 }
 ```
@@ -180,13 +180,13 @@ def verify_block_validity_proof(proof: BlockValidityProof, validator_memory: Val
     assert proof.shard_parent_block.beacon_chain_root == hash_tree_root(proof.header)
     committee = compute_committee(proof.header, validator_memory)
     # Verify that we have >=50% support
-    support_balance = sum([v.effective_balance for i, v in enumerate(committee) if get_bitfield_bit(proof.shard_bitfield, i) is True])
+    support_balance = sum([v.effective_balance for i, v in enumerate(committee) if proof.shard_bits[i]])
     total_balance = sum([v.effective_balance for i, v in enumerate(committee)])
     assert support_balance * 2 > total_balance
     # Verify shard attestations
     group_public_key = bls_aggregate_pubkeys([
         v.pubkey for v, index in enumerate(committee)
-        if get_bitfield_bit(proof.shard_bitfield, index) is True
+        if proof.shard_bits[index]
     ])
     assert bls_verify(
         pubkey=group_public_key,
@@ -196,4 +196,4 @@ def verify_block_validity_proof(proof: BlockValidityProof, validator_memory: Val
     )
 ```
 
-The size of this proof is only 200 (header) + 96 (signature) + 16 (bitfield) + 352 (shard block) = 664 bytes. It can be reduced further by replacing `ShardBlock` with `MerklePartial(lambda x: x.beacon_chain_root, ShardBlock)`, which would cut off ~220 bytes.
+The size of this proof is only 200 (header) + 96 (signature) + 16 (bits) + 352 (shard block) = 664 bytes. It can be reduced further by replacing `ShardBlock` with `MerklePartial(lambda x: x.beacon_chain_root, ShardBlock)`, which would cut off ~220 bytes.
diff --git a/specs/simple-serialize.md b/specs/simple-serialize.md
index 3318fe45b..7076b6410 100644
--- a/specs/simple-serialize.md
+++ b/specs/simple-serialize.md
@@ -15,9 +15,9 @@
         - [Default values](#default-values)
         - [Illegal types](#illegal-types)
     - [Serialization](#serialization)
-        - [`"uintN"`](#uintn)
-        - [`"bool"`](#bool)
-        - [`"null`](#null)
+        - [`uintN`](#uintn)
+        - [`boolean`](#boolean)
+        - [`null`](#null)
         - [Vectors, containers, lists, unions](#vectors-containers-lists-unions)
     - [Deserialization](#deserialization)
     - [Merkleization](#merkleization)
@@ -37,36 +37,45 @@
 ## Typing
 ### Basic types
 
-* `"uintN"`: `N`-bit unsigned integer (where `N in [8, 16, 32, 64, 128, 256]`)
-* `"bool"`: `True` or `False`
+* `uintN`: `N`-bit unsigned integer (where `N in [8, 16, 32, 64, 128, 256]`)
+* `boolean`: `True` or `False`
 
 ### Composite types
 
 * **container**: ordered heterogeneous collection of values
-    * key-pair curly bracket notation `{}`, e.g. `{"foo": "uint64", "bar": "bool"}`
-* **vector**: ordered fixed-length homogeneous collection of values
-    * angle bracket notation `[type, N]`, e.g. `["uint64", N]`
-* **list**: ordered variable-length homogeneous collection of values
-    * angle bracket notation `[type]`, e.g. `["uint64"]`
+    * python dataclass notation with key-type pairs, e.g.
+```python
+class ContainerExample(Container):
+    foo: uint64
+    bar: boolean
+```
+* **vector**: ordered fixed-length homogeneous collection, with `N` values
+    * notation `Vector[type, N]`, e.g. `Vector[uint64, N]`
+* **list**: ordered variable-length homogeneous collection, limited to `N` values
+    * notation `List[type, N]`, e.g. `List[uint64, N]`
+* **bitvector**: ordered fixed-length collection of `boolean` values, with `N` bits
+    * notation `Bitvector[N]`
+* **bitlist**: ordered variable-length collection of `boolean` values, limited to `N` bits
+    * notation `Bitlist[N]`
 * **union**: union type containing one of the given subtypes
-    * round bracket notation `(type_1, type_2, ...)`, e.g. `("null", "uint64")`
+    * notation `Union[type_1, type_2, ...]`, e.g. `union[null, uint64]`
 
 ### Variable-size and fixed-size
 
-We recursively define "variable-size" types to be lists and unions and all types that contain a variable-size type. All other types are said to be "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".
 
 ### Aliases
 
 For convenience we alias:
 
-* `"byte"` to `"uint8"` (this is a basic type)
-* `"bytes"` to `["byte"]` (this is *not* a basic type)
-* `"bytesN"` to `["byte", N]` (this is *not* a basic type)
-* `"null"`: `{}`, i.e. the empty container
+* `bit` to `boolean`
+* `byte` to `uint8` (this is a basic type)
+* `BytesN` to `Vector[byte, N]` (this is *not* a basic type)
+* `null`: `{}`, i.e. the empty container
 
 ### Default values
 
-The default value of a type upon initialization is recursively defined using `0` for `"uintN"`, `False` for `"bool"`, and `[]` for lists. Unions default to the first type in the union (with type index zero), which is `"null"` if present in the union.
+The default value of a type upon initialization is recursively defined using `0` for `uintN`, `False` for `boolean` and the elements of `Bitvector`, and `[]` for lists and `Bitlist`. Unions default to the first type in the union (with type index zero), which is `null` if present in the union.
 
 #### `is_empty`
 
@@ -74,34 +83,50 @@ An SSZ object is called empty (and thus, `is_empty(object)` returns true) if it
 
 ### Illegal types
 
-Empty vector types (i.e. `[subtype, 0]` for some `subtype`) are not legal. The `"null"` type is only legal as the first type in a union subtype (i.e. with type index zero).
+Empty vector types (i.e. `[subtype, 0]` for some `subtype`) are not legal. The `null` type is only legal as the first type in a union subtype (i.e. with type index zero).
 
 ## Serialization
 
-We recursively define the `serialize` function which consumes an object `value` (of the type specified) and returns a bytestring of type `"bytes"`.
+We recursively define the `serialize` function which consumes an object `value` (of the type specified) and returns a bytestring of type `bytes`.
 
 *Note*: In the function definitions below (`serialize`, `hash_tree_root`, `signing_root`, `is_variable_size`, etc.) objects implicitly carry their type.
 
-### `"uintN"`
+### `uintN`
 
 ```python
 assert N in [8, 16, 32, 64, 128, 256]
 return value.to_bytes(N // 8, "little")
 ```
 
-### `"bool"`
+### `boolean`
 
 ```python
 assert value in (True, False)
 return b"\x01" if value is True else b"\x00"
 ```
 
-### `"null"`
+### `null`
 
 ```python
 return b""
 ```
 
+### `Bitvector[N]`
+
+```python
+as_integer = sum([value[i] << i for i in range(len(value))])
+return as_integer.to_bytes((N + 7) // 8, "little")
+```
+
+### `Bitlist[N]`
+
+Note that from the offset coding, the length (in bytes) of the bitlist is known. An additional leading `1` bit is added so that the length in bits will also be known.
+
+```python
+as_integer = (1 << len(value)) + sum([value[i] << i for i in range(len(value))])
+return as_integer.to_bytes((as_integer.bit_length() + 7) // 8, "little")
+```
+
 ### Vectors, containers, lists, unions
 
 ```python
@@ -136,23 +161,47 @@ return serialized_type_index + serialized_bytes
 
 Because serialization is an injective function (i.e. two distinct objects of the same type will serialize to different values) any bytestring has at most one object it could deserialize to. Efficient algorithms for computing this object can be found in [the implementations](#implementations).
 
+Note that deserialization requires hardening against invalid inputs. A non-exhaustive list:
+- Offsets: out of order, out of range, mismatching minimum element size
+- Scope: Extra unused bytes, not aligned with element size.
+- More elements than a list limit allows. Part of enforcing consensus.
+
 ## Merkleization
 
 We first define helper functions:
 
 * `pack`: Given ordered objects of the same basic type, serialize them, pack them into `BYTES_PER_CHUNK`-byte chunks, right-pad the last chunk with zero bytes, and return the chunks.
-* `merkleize`: Given ordered `BYTES_PER_CHUNK`-byte chunks, if necessary append zero chunks so that the number of chunks is a power of two, Merkleize the chunks, and return the root. Note that `merkleize` on a single chunk is simply that chunk, i.e. the identity when the number of chunks is one.
+* `next_pow_of_two(i)`: get the next power of 2 of `i`, if not already a power of 2, with 0 mapping to 1. Examples: `0->1, 1->1, 2->2, 3->4, 4->4, 6->8, 9->16`
+* `merkleize(data, pad_for)`: Given ordered `BYTES_PER_CHUNK`-byte chunks, if necessary append zero chunks so that the number of chunks is a power of two, Merkleize the chunks, and return the root.
+  The merkleization depends on the effective input, which can be padded: if `pad_for=L`, then pad the `data` with zeroed chunks to `next_pow_of_two(L)` (virtually for memory efficiency).
+  Then, merkleize the chunks (empty input is padded to 1 zero chunk):
+    - If `1` chunk: A single chunk is simply that chunk, i.e. the identity when the number of chunks is one.
+    - If `> 1` chunks: pad to `next_pow_of_two(len(chunks))`, merkleize as binary tree.
 * `mix_in_length`: Given a Merkle root `root` and a length `length` (`"uint256"` little-endian serialization) return `hash(root + length)`.
 * `mix_in_type`: Given a Merkle root `root` and a type_index `type_index` (`"uint256"` little-endian serialization) return `hash(root + type_index)`.
 
 We now define Merkleization `hash_tree_root(value)` of an object `value` recursively:
 
 * `merkleize(pack(value))` if `value` is a basic object or a vector of basic objects
-* `mix_in_length(merkleize(pack(value)), len(value))` if `value` is a list of basic objects
+* `mix_in_length(merkleize(pack(value), pad_for=(N * elem_size / BYTES_PER_CHUNK)), len(value))` if `value` is a list of basic objects.
 * `merkleize([hash_tree_root(element) for element in value])` if `value` is a vector of composite objects or a container
-* `mix_in_length(merkleize([hash_tree_root(element) for element in value]), len(value))` if `value` is a list of composite objects
+* `mix_in_length(merkleize([hash_tree_root(element) for element in value], pad_for=N), len(value))` if `value` is a list of composite objects.
 * `mix_in_type(merkleize(value.value), value.type_index)` if `value` is of union type
 
+### Merkleization of `Bitvector[N]`
+
+```python
+as_integer = sum([value[i] << i for i in range(len(value))])
+return merkleize(as_integer.to_bytes((N + 7) // 8, "little"))
+```
+
+### `Bitlist[N]`
+
+```python
+as_integer = sum([value[i] << i for i in range(len(value))])
+return mix_in_length(merkleize(as_integer.to_bytes((N + 7) // 8, "little")), len(value))
+```
+
 ## Self-signed containers
 
 Let `value` be a self-signed container object. The convention is that the signature (e.g. a `"bytes96"` BLS12-381 signature) be the last field of `value`. Further, the signed message for `value` is `signing_root(value) = hash_tree_root(truncate_last(value))` where `truncate_last` truncates the last element of `value`.
diff --git a/specs/validator/0_beacon-chain-validator.md b/specs/validator/0_beacon-chain-validator.md
index 44b5e2a3c..7d5630c7b 100644
--- a/specs/validator/0_beacon-chain-validator.md
+++ b/specs/validator/0_beacon-chain-validator.md
@@ -44,8 +44,8 @@
                 - [Crosslink vote](#crosslink-vote)
             - [Construct attestation](#construct-attestation)
                 - [Data](#data)
-                - [Aggregation bitfield](#aggregation-bitfield)
-                - [Custody bitfield](#custody-bitfield)
+                - [Aggregation bits](#aggregation-bits)
+                - [Custody bits](#custody-bits)
                 - [Aggregate signature](#aggregate-signature)
     - [How to avoid slashing](#how-to-avoid-slashing)
         - [Proposer slashing](#proposer-slashing)
@@ -329,19 +329,15 @@ Next, the validator creates `attestation`, an [`Attestation`](../core/0_beacon-c
 
 Set `attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object defined in the previous section, [attestation data](#attestation-data).
 
-##### Aggregation bitfield
+##### Aggregation bits
 
-* Let `aggregation_bitfield` be a byte array filled with zeros of length `(len(committee) + 7) // 8`.
-* Let `index_into_committee` be the index into the validator's `committee` at which `validator_index` is located.
-* Set `aggregation_bitfield[index_into_committee // 8] |= 2 ** (index_into_committee % 8)`.
-* Set `attestation.aggregation_bitfield = aggregation_bitfield`.
+* Let `attestation.aggregation_bits` be a `Bitlist[MAX_INDICES_PER_ATTESTATION]` where the bits at the index in the aggregated validator's `committee` is set to `0b1`.
 
-*Note*: Calling `get_attesting_indices(state, attestation.data, attestation.aggregation_bitfield)` should return a list of length equal to 1, containing `validator_index`.
+*Note*: Calling `get_attesting_indices(state, attestation.data, attestation.aggregation_bits)` should return a list of length equal to 1, containing `validator_index`.
 
-##### Custody bitfield
+##### Custody bits
 
-* Let `custody_bitfield` be a byte array filled with zeros of length `(len(committee) + 7) // 8`.
-* Set `attestation.custody_bitfield = custody_bitfield`.
+* Let `attestation.custody_bits` be a `Bitlist[MAX_INDICES_PER_ATTESTATION]` filled with zeros of length `len(committee)`.
 
 *Note*: This is a stub for Phase 0.
 
diff --git a/specs/validator/beacon_node_oapi.yaml b/specs/validator/beacon_node_oapi.yaml
index 74be21fac..4da8f7933 100644
--- a/specs/validator/beacon_node_oapi.yaml
+++ b/specs/validator/beacon_node_oapi.yaml
@@ -415,16 +415,16 @@ components:
             type: object
             description: "The [`Attestation`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestation) object from the Eth2.0 spec."
             properties:
-              aggregation_bitfield:
+              aggregation_bits:
                 type: string
                 format: byte
                 pattern: "^0x[a-fA-F0-9]+$"
-                description: "Attester aggregation bitfield."
-              custody_bitfield:
+                description: "Attester aggregation bits."
+              custody_bits:
                 type: string
                 format: byte
                 pattern: "^0x[a-fA-F0-9]+$"
-                description: "Custody bitfield."
+                description: "Custody bits."
               signature:
                 type: string
                 format: byte
diff --git a/test_libs/pyspec/eth2spec/debug/decode.py b/test_libs/pyspec/eth2spec/debug/decode.py
index c0b53b0ef..c0b977ab3 100644
--- a/test_libs/pyspec/eth2spec/debug/decode.py
+++ b/test_libs/pyspec/eth2spec/debug/decode.py
@@ -1,13 +1,13 @@
 from typing import Any
 from eth2spec.utils.ssz.ssz_impl import hash_tree_root
 from eth2spec.utils.ssz.ssz_typing import (
-    SSZType, SSZValue, uint, Container, Bytes, List, Bool,
+    SSZType, SSZValue, uint, Container, Bytes, List, boolean,
     Vector, BytesN
 )
 
 
 def decode(data: Any, typ: SSZType) -> SSZValue:
-    if issubclass(typ, (uint, Bool)):
+    if issubclass(typ, (uint, boolean)):
         return typ(data)
     elif issubclass(typ, (List, Vector)):
         return typ(decode(element, typ.elem_type) for element in data)
diff --git a/test_libs/pyspec/eth2spec/debug/encode.py b/test_libs/pyspec/eth2spec/debug/encode.py
index 02814e441..670f580b2 100644
--- a/test_libs/pyspec/eth2spec/debug/encode.py
+++ b/test_libs/pyspec/eth2spec/debug/encode.py
@@ -1,6 +1,6 @@
 from eth2spec.utils.ssz.ssz_impl import hash_tree_root
 from eth2spec.utils.ssz.ssz_typing import (
-    SSZValue, uint, Container, Bool
+    SSZValue, uint, Container, boolean
 )
 
 
@@ -10,7 +10,7 @@ def encode(value: SSZValue, include_hash_tree_roots=False):
         if value.type().byte_len > 8:
             return str(int(value))
         return int(value)
-    elif isinstance(value, Bool):
+    elif isinstance(value, boolean):
         return value == 1
     elif isinstance(value, list):  # normal python lists, ssz-List, Vector
         return [encode(element, include_hash_tree_roots) for element in value]
diff --git a/test_libs/pyspec/eth2spec/debug/random_value.py b/test_libs/pyspec/eth2spec/debug/random_value.py
index c6efb722b..95a3ae970 100644
--- a/test_libs/pyspec/eth2spec/debug/random_value.py
+++ b/test_libs/pyspec/eth2spec/debug/random_value.py
@@ -2,8 +2,8 @@ from random import Random
 from enum import Enum
 
 from eth2spec.utils.ssz.ssz_typing import (
-    SSZType, SSZValue, BasicValue, BasicType, uint, Container, Bytes, List, Bool,
-    Vector, BytesN
+    SSZType, SSZValue, BasicValue, BasicType, uint, Container, Bytes, List, boolean,
+    Vector, BytesN, Bitlist, Bitvector
 )
 
 # in bytes
@@ -83,12 +83,12 @@ def get_random_ssz_object(rng: Random,
             return get_max_basic_value(typ)
         else:
             return get_random_basic_value(rng, typ)
-    elif issubclass(typ, Vector):
+    elif issubclass(typ, Vector) or issubclass(typ, Bitvector):
         return typ(
             get_random_ssz_object(rng, typ.elem_type, max_bytes_length, max_list_length, mode, chaos)
             for _ in range(typ.length)
         )
-    elif issubclass(typ, List):
+    elif issubclass(typ, List) or issubclass(typ, Bitlist):
         length = rng.randint(0, min(typ.length, max_list_length))
         if mode == RandomizationMode.mode_one_count:
             length = 1
@@ -118,7 +118,7 @@ def get_random_bytes_list(rng: Random, length: int) -> bytes:
 
 
 def get_random_basic_value(rng: Random, typ: BasicType) -> BasicValue:
-    if issubclass(typ, Bool):
+    if issubclass(typ, boolean):
         return typ(rng.choice((True, False)))
     elif issubclass(typ, uint):
         assert typ.byte_len in UINT_BYTE_SIZES
@@ -128,7 +128,7 @@ def get_random_basic_value(rng: Random, typ: BasicType) -> BasicValue:
 
 
 def get_min_basic_value(typ: BasicType) -> BasicValue:
-    if issubclass(typ, Bool):
+    if issubclass(typ, boolean):
         return typ(False)
     elif issubclass(typ, uint):
         assert typ.byte_len in UINT_BYTE_SIZES
@@ -138,7 +138,7 @@ def get_min_basic_value(typ: BasicType) -> BasicValue:
 
 
 def get_max_basic_value(typ: BasicType) -> BasicValue:
-    if issubclass(typ, Bool):
+    if issubclass(typ, boolean):
         return typ(True)
     elif issubclass(typ, uint):
         assert typ.byte_len in UINT_BYTE_SIZES
diff --git a/test_libs/pyspec/eth2spec/fuzzing/decoder.py b/test_libs/pyspec/eth2spec/fuzzing/decoder.py
index e533ca5c2..ccca17385 100644
--- a/test_libs/pyspec/eth2spec/fuzzing/decoder.py
+++ b/test_libs/pyspec/eth2spec/fuzzing/decoder.py
@@ -18,8 +18,15 @@ def translate_typ(typ) -> ssz.BaseSedes:
     elif issubclass(typ, spec_ssz.Vector):
         return ssz.Vector(translate_typ(typ.elem_type), typ.length)
     elif issubclass(typ, spec_ssz.List):
+        # TODO: Make py-ssz List support the new fixed length list
         return ssz.List(translate_typ(typ.elem_type))
-    elif issubclass(typ, spec_ssz.Bool):
+    elif issubclass(typ, spec_ssz.Bitlist):
+        # TODO: Once Bitlist implemented in py-ssz, use appropriate type
+        return ssz.List(translate_typ(typ.elem_type))
+    elif issubclass(typ, spec_ssz.Bitvector):
+        # TODO: Once Bitvector implemented in py-ssz, use appropriate type
+        return ssz.Vector(translate_typ(typ.elem_type), typ.length)
+    elif issubclass(typ, spec_ssz.boolean):
         return ssz.boolean
     elif issubclass(typ, spec_ssz.uint):
         if typ.byte_len == 1:
@@ -64,10 +71,14 @@ def translate_value(value, typ):
             raise TypeError("invalid uint size")
     elif issubclass(typ, spec_ssz.List):
         return [translate_value(elem, typ.elem_type) for elem in value]
-    elif issubclass(typ, spec_ssz.Bool):
+    elif issubclass(typ, spec_ssz.boolean):
         return value
     elif issubclass(typ, spec_ssz.Vector):
         return typ(*(translate_value(elem, typ.elem_type) for elem in value))
+    elif issubclass(typ, spec_ssz.Bitlist):
+        return typ(value)
+    elif issubclass(typ, spec_ssz.Bitvector):
+        return typ(value)
     elif issubclass(typ, spec_ssz.BytesN):
         return typ(value)
     elif issubclass(typ, spec_ssz.Bytes):
diff --git a/test_libs/pyspec/eth2spec/fuzzing/test_decoder.py b/test_libs/pyspec/eth2spec/fuzzing/test_decoder.py
index 26ee6e913..ea1f1d47f 100644
--- a/test_libs/pyspec/eth2spec/fuzzing/test_decoder.py
+++ b/test_libs/pyspec/eth2spec/fuzzing/test_decoder.py
@@ -9,7 +9,9 @@ def test_decoder():
     rng = Random(123)
 
     # check these types only, Block covers a lot of operation types already.
-    for typ in [spec.BeaconBlock, spec.BeaconState, spec.IndexedAttestation, spec.AttestationDataAndCustodyBit]:
+    # TODO: Once has Bitlists and Bitvectors, add back
+    #       spec.BeaconState and spec.BeaconBlock
+    for typ in [spec.IndexedAttestation, spec.AttestationDataAndCustodyBit]:
         # create a random pyspec value
         original = random_value.get_random_ssz_object(rng, typ, 100, 10,
                                                       mode=random_value.RandomizationMode.mode_random,
diff --git a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
index eb1e9a9ed..5adb022a6 100644
--- a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
+++ b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
@@ -118,5 +118,5 @@ def test_on_attestation_invalid_attestation(spec, state):
 
     attestation = get_valid_attestation(spec, state, slot=block.slot)
     # make attestation invalid
-    attestation.custody_bitfield = b'\xf0' + attestation.custody_bitfield[1:]
+    attestation.custody_bits[0:8] = [0, 0, 0, 0, 1, 1, 1, 1]
     run_on_attestation(spec, state, store, attestation, False)
diff --git a/test_libs/pyspec/eth2spec/test/helpers/attestations.py b/test_libs/pyspec/eth2spec/test/helpers/attestations.py
index d717bdc0d..ffd8d1afa 100644
--- a/test_libs/pyspec/eth2spec/test/helpers/attestations.py
+++ b/test_libs/pyspec/eth2spec/test/helpers/attestations.py
@@ -1,10 +1,10 @@
 from typing import List
 
-from eth2spec.test.helpers.bitfields import set_bitfield_bit
 from eth2spec.test.helpers.block import build_empty_block_for_next_slot, sign_block
 from eth2spec.test.helpers.keys import privkeys
 from eth2spec.utils.bls import bls_sign, bls_aggregate_signatures
 from eth2spec.utils.ssz.ssz_impl import hash_tree_root
+from eth2spec.utils.ssz.ssz_typing import Bitlist
 
 
 def build_attestation_data(spec, state, slot, shard):
@@ -67,13 +67,12 @@ def get_valid_attestation(spec, state, slot=None, signed=False):
     )
 
     committee_size = len(crosslink_committee)
-    bitfield_length = (committee_size + 7) // 8
-    aggregation_bitfield = b'\x00' * bitfield_length
-    custody_bitfield = b'\x00' * bitfield_length
+    aggregation_bits = Bitlist[spec.MAX_VALIDATORS_PER_COMMITTEE](*([0] * committee_size))
+    custody_bits = Bitlist[spec.MAX_VALIDATORS_PER_COMMITTEE](*([0] * committee_size))
     attestation = spec.Attestation(
-        aggregation_bitfield=aggregation_bitfield,
+        aggregation_bits=aggregation_bits,
         data=attestation_data,
-        custody_bitfield=custody_bitfield,
+        custody_bits=custody_bits,
     )
     fill_aggregate_attestation(spec, state, attestation)
     if signed:
@@ -106,7 +105,7 @@ def sign_attestation(spec, state, attestation):
     participants = spec.get_attesting_indices(
         state,
         attestation.data,
-        attestation.aggregation_bitfield,
+        attestation.aggregation_bits,
     )
 
     attestation.signature = sign_aggregate_attestation(spec, state, attestation.data, participants)
@@ -136,7 +135,7 @@ def fill_aggregate_attestation(spec, state, attestation):
         attestation.data.crosslink.shard,
     )
     for i in range(len(crosslink_committee)):
-        attestation.aggregation_bitfield = set_bitfield_bit(attestation.aggregation_bitfield, i)
+        attestation.aggregation_bits[i] = True
 
 
 def add_attestation_to_state(spec, state, attestation, slot):
diff --git a/test_libs/pyspec/eth2spec/test/helpers/bitfields.py b/test_libs/pyspec/eth2spec/test/helpers/bitfields.py
deleted file mode 100644
index 50e5b6cba..000000000
--- a/test_libs/pyspec/eth2spec/test/helpers/bitfields.py
+++ /dev/null
@@ -1,11 +0,0 @@
-def set_bitfield_bit(bitfield, i):
-    """
-    Set the bit in ``bitfield`` at position ``i`` to ``1``.
-    """
-    byte_index = i // 8
-    bit_index = i % 8
-    return (
-        bitfield[:byte_index] +
-        bytes([bitfield[byte_index] | (1 << bit_index)]) +
-        bitfield[byte_index + 1:]
-    )
diff --git a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py
index d4e22decf..df2f0627f 100644
--- a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py
+++ b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py
@@ -1,5 +1,3 @@
-from copy import deepcopy
-
 from eth2spec.test.context import spec_state_test, expect_assertion_error, always_bls, with_all_phases, with_phases
 from eth2spec.test.helpers.attestations import (
     get_valid_attestation,
@@ -10,6 +8,7 @@ from eth2spec.test.helpers.state import (
     next_slot,
 )
 from eth2spec.test.helpers.block import apply_empty_block
+from eth2spec.utils.ssz.ssz_typing import Bitlist
 
 
 def run_attestation_processing(spec, state, attestation, valid=True):
@@ -274,11 +273,14 @@ def test_bad_crosslink_end_epoch(spec, state):
 
 @with_all_phases
 @spec_state_test
-def test_inconsistent_bitfields(spec, state):
+def test_inconsistent_bits(spec, state):
     attestation = get_valid_attestation(spec, state)
     state.slot += spec.MIN_ATTESTATION_INCLUSION_DELAY
 
-    attestation.custody_bitfield = deepcopy(attestation.aggregation_bitfield) + b'\x00'
+    custody_bits = attestation.aggregation_bits[:]
+    custody_bits.append(False)
+
+    attestation.custody_bits = custody_bits
 
     sign_attestation(spec, state, attestation)
 
@@ -287,11 +289,11 @@ def test_inconsistent_bitfields(spec, state):
 
 @with_phases(['phase0'])
 @spec_state_test
-def test_non_empty_custody_bitfield(spec, state):
+def test_non_empty_custody_bits(spec, state):
     attestation = get_valid_attestation(spec, state)
     state.slot += spec.MIN_ATTESTATION_INCLUSION_DELAY
 
-    attestation.custody_bitfield = deepcopy(attestation.aggregation_bitfield)
+    attestation.custody_bits = attestation.aggregation_bits[:]
 
     sign_attestation(spec, state, attestation)
 
@@ -300,11 +302,12 @@ def test_non_empty_custody_bitfield(spec, state):
 
 @with_all_phases
 @spec_state_test
-def test_empty_aggregation_bitfield(spec, state):
+def test_empty_aggregation_bits(spec, state):
     attestation = get_valid_attestation(spec, state)
     state.slot += spec.MIN_ATTESTATION_INCLUSION_DELAY
 
-    attestation.aggregation_bitfield = b'\x00' * len(attestation.aggregation_bitfield)
+    attestation.aggregation_bits = Bitlist[spec.MAX_VALIDATORS_PER_COMMITTEE](
+        *([0b0] * len(attestation.aggregation_bits)))
 
     sign_attestation(spec, state, attestation)
 
diff --git a/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py b/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
index b9c7b6d38..d5855a755 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
@@ -1,7 +1,8 @@
 from ..merkle_minimal import merkleize_chunks
 from ..hash_function import hash
 from .ssz_typing import (
-    SSZValue, SSZType, BasicValue, BasicType, Series, Elements, Bool, Container, List, Bytes, uint,
+    SSZValue, SSZType, BasicValue, BasicType, Series, Elements, Bits, boolean, Container, List, Bytes,
+    Bitlist, Bitvector, uint,
 )
 
 # SSZ Serialization
@@ -13,7 +14,7 @@ BYTES_PER_LENGTH_OFFSET = 4
 def serialize_basic(value: SSZValue):
     if isinstance(value, uint):
         return value.to_bytes(value.type().byte_len, 'little')
-    elif isinstance(value, Bool):
+    elif isinstance(value, boolean):
         if value:
             return b'\x01'
         else:
@@ -25,7 +26,7 @@ def serialize_basic(value: SSZValue):
 def deserialize_basic(value, typ: BasicType):
     if issubclass(typ, uint):
         return typ(int.from_bytes(value, 'little'))
-    elif issubclass(typ, Bool):
+    elif issubclass(typ, boolean):
         assert value in (b'\x00', b'\x01')
         return typ(value == b'\x01')
     else:
@@ -39,6 +40,12 @@ def is_empty(obj: SSZValue):
 def serialize(obj: SSZValue):
     if isinstance(obj, BasicValue):
         return serialize_basic(obj)
+    elif isinstance(obj, Bitvector):
+        as_integer = sum([obj[i] << i for i in range(len(obj))])
+        return as_integer.to_bytes((len(obj) + 7) // 8, "little")
+    elif isinstance(obj, Bitlist):
+        as_integer = (1 << len(obj)) + sum([obj[i] << i for i in range(len(obj))])
+        return as_integer.to_bytes((as_integer.bit_length() + 7) // 8, "little")
     elif isinstance(obj, Series):
         return encode_series(obj)
     else:
@@ -85,6 +92,12 @@ def encode_series(values: Series):
 def pack(values: Series):
     if isinstance(values, bytes):  # Bytes and BytesN are already packed
         return values
+    elif isinstance(values, Bitvector):
+        as_integer = sum([values[i] << i for i in range(len(values))])
+        return as_integer.to_bytes((values.length + 7) // 8, "little")
+    elif isinstance(values, Bitlist):
+        as_integer = sum([values[i] << i for i in range(len(values))])
+        return as_integer.to_bytes((values.length + 7) // 8, "little")
     return b''.join([serialize_basic(value) for value in values])
 
 
@@ -115,6 +128,8 @@ def item_length(typ: SSZType) -> int:
 def chunk_count(typ: SSZType) -> int:
     if isinstance(typ, BasicType):
         return 1
+    elif issubclass(typ, Bits):
+        return (typ.length + 255) // 256
     elif issubclass(typ, Elements):
         return (typ.length * item_length(typ.elem_type) + 31) // 32
     elif issubclass(typ, Container):
@@ -134,7 +149,7 @@ def hash_tree_root(obj: SSZValue):
     else:
         raise Exception(f"Type not supported: {type(obj)}")
 
-    if isinstance(obj, (List, Bytes)):
+    if isinstance(obj, (List, Bytes, Bitlist)):
         return mix_in_length(merkleize_chunks(leaves, pad_to=chunk_count(obj.type())), len(obj))
     else:
         return merkleize_chunks(leaves)
diff --git a/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py b/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
index 58e66ca68..d87a22399 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
@@ -31,7 +31,7 @@ class BasicValue(int, SSZValue, metaclass=BasicType):
     pass
 
 
-class Bool(BasicValue):  # can't subclass bool.
+class boolean(BasicValue):  # can't subclass bool.
     byte_len = 1
 
     def __new__(cls, value: int):  # int value, but can be any subclass of int (bool, Bit, Bool, etc...)
@@ -48,7 +48,7 @@ class Bool(BasicValue):  # can't subclass bool.
 
 
 # Alias for Bool
-class Bit(Bool):
+class bit(boolean):
     pass
 
 
@@ -233,7 +233,7 @@ class ParamsMeta(SSZType):
         return f"{self.__name__}~{self.__class__.__name__}"
 
     def __repr__(self):
-        return self, self.__class__
+        return f"{self.__name__}~{self.__class__.__name__}"
 
     def attr_from_params(self, p):
         # single key params are valid too. Wrap them in a tuple.
@@ -310,6 +310,10 @@ class BaseList(list, Elements):
         cls = self.__class__
         return f"{cls.__name__}[{cls.elem_type.__name__}, {cls.length}]({', '.join(str(v) for v in self)})"
 
+    def __repr__(self):
+        cls = self.__class__
+        return f"{cls.__name__}[{cls.elem_type.__name__}, {cls.length}]({', '.join(str(v) for v in self)})"
+
     def __getitem__(self, k) -> SSZValue:
         if isinstance(k, int):  # check if we are just doing a lookup, and not slicing
             if k < 0:
@@ -320,12 +324,18 @@ class BaseList(list, Elements):
         return super().__getitem__(k)
 
     def __setitem__(self, k, v):
-        if k < 0:
-            raise IndexError(f"cannot set item in type {self.__class__} at negative index {k} (to {v})")
-        if k > len(self):
-            raise IndexError(f"cannot set item in type {self.__class__}"
-                             f" at out of bounds index {k} (to {v}, bound: {len(self)})")
-        super().__setitem__(k, coerce_type_maybe(v, self.__class__.elem_type, strict=True))
+        if type(k) == slice:
+            if (k.start is not None and k.start < 0) or (k.stop is not None and k.stop > len(self)):
+                raise IndexError(f"cannot set item in type {self.__class__}"
+                                 f" at out of bounds slice {k} (to {v}, bound: {len(self)})")
+            super().__setitem__(k, [coerce_type_maybe(x, self.__class__.elem_type) for x in v])
+        else:
+            if k < 0:
+                raise IndexError(f"cannot set item in type {self.__class__} at negative index {k} (to {v})")
+            if k > len(self):
+                raise IndexError(f"cannot set item in type {self.__class__}"
+                                 f" at out of bounds index {k} (to {v}, bound: {len(self)})")
+            super().__setitem__(k, coerce_type_maybe(v, self.__class__.elem_type, strict=True))
 
     def append(self, v):
         super().append(coerce_type_maybe(v, self.__class__.elem_type, strict=True))
@@ -338,6 +348,48 @@ class BaseList(list, Elements):
         return self[len(self) - 1]
 
 
+class BitElementsType(ElementsType):
+    elem_type: SSZType = boolean
+    length: int
+
+
+class Bits(BaseList, metaclass=BitElementsType):
+    pass
+
+
+class Bitlist(Bits):
+    @classmethod
+    def is_fixed_size(cls):
+        return False
+
+    @classmethod
+    def default(cls):
+        return cls()
+
+
+class Bitvector(Bits):
+
+    @classmethod
+    def extract_args(cls, *args):
+        if len(args) == 0:
+            return cls.default()
+        else:
+            return super().extract_args(*args)
+
+    @classmethod
+    def value_check(cls, value):
+        # check length limit strictly
+        return len(value) == cls.length and super().value_check(value)
+
+    @classmethod
+    def is_fixed_size(cls):
+        return True
+
+    @classmethod
+    def default(cls):
+        return cls(0 for _ in range(cls.length))
+
+
 class List(BaseList):
 
     @classmethod
diff --git a/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_impl.py b/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_impl.py
index 82fb4ec68..637d9c5c4 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_impl.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_impl.py
@@ -1,7 +1,8 @@
 from typing import Iterable
 from .ssz_impl import serialize, hash_tree_root
 from .ssz_typing import (
-    Bit, Bool, Container, List, Vector, Bytes, BytesN,
+    bit, boolean, Container, List, Vector, Bytes, BytesN,
+    Bitlist, Bitvector,
     uint8, uint16, uint32, uint64, uint256, byte
 )
 from ..hash_function import hash as bytes_hash
@@ -74,10 +75,32 @@ def merge(a: str, branch: Iterable[str]) -> str:
 
 
 test_data = [
-    ("bit F", Bit(False), "00", chunk("00")),
-    ("bit T", Bit(True), "01", chunk("01")),
-    ("bool F", Bool(False), "00", chunk("00")),
-    ("bool T", Bool(True), "01", chunk("01")),
+    ("bit F", bit(False), "00", chunk("00")),
+    ("bit T", bit(True), "01", chunk("01")),
+    ("boolean F", boolean(False), "00", chunk("00")),
+    ("boolean T", boolean(True), "01", chunk("01")),
+    ("bitvector TTFTFTFF", Bitvector[8](1, 1, 0, 1, 0, 1, 0, 0), "2b", chunk("2b")),
+    ("bitlist TTFTFTFF", Bitlist[8](1, 1, 0, 1, 0, 1, 0, 0), "2b01", h(chunk("2b"), chunk("08"))),
+    ("bitvector FTFT", Bitvector[4](0, 1, 0, 1), "0a", chunk("0a")),
+    ("bitlist FTFT", Bitlist[4](0, 1, 0, 1), "1a", h(chunk("0a"), chunk("04"))),
+    ("bitvector FTF", Bitvector[3](0, 1, 0), "02", chunk("02")),
+    ("bitlist FTF", Bitlist[3](0, 1, 0), "0a", h(chunk("02"), chunk("03"))),
+    ("bitvector TFTFFFTTFT", Bitvector[10](1, 0, 1, 0, 0, 0, 1, 1, 0, 1), "c502", chunk("c502")),
+    ("bitlist TFTFFFTTFT", Bitlist[16](1, 0, 1, 0, 0, 0, 1, 1, 0, 1), "c506", h(chunk("c502"), chunk("0A"))),
+    ("bitvector TFTFFFTTFTFFFFTT", Bitvector[16](1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1, 1),
+     "c5c2", chunk("c5c2")),
+    ("bitlist TFTFFFTTFTFFFFTT", Bitlist[16](1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1, 1),
+     "c5c201", h(chunk("c5c2"), chunk("10"))),
+    ("long bitvector", Bitvector[512](1 for i in range(512)),
+     "ff" * 64, h("ff" * 32, "ff" * 32)),
+    ("long bitlist", Bitlist[512](1),
+     "03", h(h(chunk("01"), chunk("")), chunk("01"))),
+    ("long bitlist", Bitlist[512](1 for i in range(512)),
+     "ff" * 64 + "01", h(h("ff" * 32, "ff" * 32), chunk("0002"))),
+    ("odd bitvector", Bitvector[513](1 for i in range(513)),
+     "ff" * 64 + "01", h(h("ff" * 32, "ff" * 32), h(chunk("01"), chunk("")))),
+    ("odd bitlist", Bitlist[513](1 for i in range(513)),
+     "ff" * 64 + "03", h(h(h("ff" * 32, "ff" * 32), h(chunk("01"), chunk(""))), chunk("0102"))),
     ("uint8 00", uint8(0x00), "00", chunk("00")),
     ("uint8 01", uint8(0x01), "01", chunk("01")),
     ("uint8 ab", uint8(0xab), "ab", chunk("ab")),
diff --git a/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_typing.py b/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_typing.py
index 2af742360..f746a29c9 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_typing.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/test_ssz_typing.py
@@ -1,6 +1,6 @@
 from .ssz_typing import (
     SSZValue, SSZType, BasicValue, BasicType, Series, ElementsType,
-    Elements, Bit, Bool, Container, List, Vector, Bytes, BytesN,
+    Elements, bit, boolean, Container, List, Vector, Bytes, BytesN,
     byte, uint, uint8, uint16, uint32, uint64, uint128, uint256,
     Bytes32, Bytes48
 )
@@ -22,8 +22,8 @@ def test_subclasses():
         assert issubclass(u, SSZValue)
         assert isinstance(u, SSZType)
         assert isinstance(u, BasicType)
-    assert issubclass(Bool, BasicValue)
-    assert isinstance(Bool, BasicType)
+    assert issubclass(boolean, BasicValue)
+    assert isinstance(boolean, BasicType)
 
     for c in [Container, List, Vector, Bytes, BytesN]:
         assert issubclass(c, Series)
@@ -45,16 +45,16 @@ def test_basic_instances():
         assert isinstance(v, BasicValue)
         assert isinstance(v, SSZValue)
 
-    assert isinstance(Bool(True), BasicValue)
-    assert isinstance(Bool(False), BasicValue)
-    assert isinstance(Bit(True), Bool)
-    assert isinstance(Bit(False), Bool)
+    assert isinstance(boolean(True), BasicValue)
+    assert isinstance(boolean(False), BasicValue)
+    assert isinstance(bit(True), boolean)
+    assert isinstance(bit(False), boolean)
 
 
 def test_basic_value_bounds():
     max = {
-        Bool: 2 ** 1,
-        Bit: 2 ** 1,
+        boolean: 2 ** 1,
+        bit: 2 ** 1,
         uint8: 2 ** (8 * 1),
         byte: 2 ** (8 * 1),
         uint16: 2 ** (8 * 2),