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..67e5914d1 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
@@ -261,12 +266,12 @@ 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 +291,20 @@ 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
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)