diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 7bf0c2d99..374bac5cf 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -37,6 +37,7 @@
     - [`get_base_reward_per_increment`](#get_base_reward_per_increment)
     - [`get_base_reward`](#get_base_reward)
     - [`get_unslashed_participating_indices`](#get_unslashed_participating_indices)
+    - [`get_attestation_participation_flag_indices`](#get_attestation_participation_flag_indices)
     - [`get_flag_index_deltas`](#get_flag_index_deltas)
     - [Modified `get_inactivity_penalty_deltas`](#modified-get_inactivity_penalty_deltas)
   - [Beacon state mutators](#beacon-state-mutators)
@@ -352,6 +353,37 @@ def get_unslashed_participating_indices(state: BeaconState, flag_index: int, epo
     return set(filter(lambda index: not state.validators[index].slashed, participating_indices))
 ```
 
+#### `get_attestation_participation_flag_indices`
+
+```python
+def get_attestation_participation_flag_indices(state: BeaconState,
+                                               data: AttestationData,
+                                               inclusion_delay: uint64) -> Sequence[int]:
+    """
+    Return the flag indices that are satisfied by an attestation.
+    """
+    if data.target.epoch == get_current_epoch(state):
+        justified_checkpoint = state.current_justified_checkpoint
+    else:
+        justified_checkpoint = state.previous_justified_checkpoint
+
+    # Matching roots
+    is_matching_source = data.source == justified_checkpoint
+    is_matching_target = is_matching_source and data.target.root == get_block_root(state, data.target.epoch)
+    is_matching_head = is_matching_target and data.beacon_block_root == get_block_root_at_slot(state, data.slot)
+    assert is_matching_source
+
+    participation_flag_indices = []
+    if is_matching_source and inclusion_delay <= integer_squareroot(SLOTS_PER_EPOCH):
+        participation_flag_indices.append(TIMELY_SOURCE_FLAG_INDEX)
+    if is_matching_target and inclusion_delay <= SLOTS_PER_EPOCH:
+        participation_flag_indices.append(TIMELY_TARGET_FLAG_INDEX)
+    if is_matching_head and inclusion_delay == MIN_ATTESTATION_INCLUSION_DELAY:
+        participation_flag_indices.append(TIMELY_HEAD_FLAG_INDEX)
+
+    return participation_flag_indices
+```
+
 #### `get_flag_index_deltas`
 
 ```python
@@ -463,32 +495,18 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     committee = get_beacon_committee(state, data.slot, data.index)
     assert len(attestation.aggregation_bits) == len(committee)
 
-    if data.target.epoch == get_current_epoch(state):
-        epoch_participation = state.current_epoch_participation
-        justified_checkpoint = state.current_justified_checkpoint
-    else:
-        epoch_participation = state.previous_epoch_participation
-        justified_checkpoint = state.previous_justified_checkpoint
-
-    # Matching roots
-    is_matching_source = data.source == justified_checkpoint
-    is_matching_target = is_matching_source and data.target.root == get_block_root(state, data.target.epoch)
-    is_matching_head = is_matching_target and data.beacon_block_root == get_block_root_at_slot(state, data.slot)
-    assert is_matching_source
+    # Participation flag indices
+    participation_flag_indices = get_attestation_participation_flag_indices(state, data, state.slot - data.slot)
 
     # Verify signature
     assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))
 
-    # Participation flag indices
-    participation_flag_indices = []
-    if is_matching_source and state.slot <= data.slot + integer_squareroot(SLOTS_PER_EPOCH):
-        participation_flag_indices.append(TIMELY_SOURCE_FLAG_INDEX)
-    if is_matching_target and state.slot <= data.slot + SLOTS_PER_EPOCH:
-        participation_flag_indices.append(TIMELY_TARGET_FLAG_INDEX)
-    if is_matching_head and state.slot == data.slot + MIN_ATTESTATION_INCLUSION_DELAY:
-        participation_flag_indices.append(TIMELY_HEAD_FLAG_INDEX)
-
     # Update epoch participation flags
+    if data.target.epoch == get_current_epoch(state):
+        epoch_participation = state.current_epoch_participation
+    else:
+        epoch_participation = state.previous_epoch_participation
+
     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():
diff --git a/specs/altair/fork.md b/specs/altair/fork.md
index be562b82b..d8374d009 100644
--- a/specs/altair/fork.md
+++ b/specs/altair/fork.md
@@ -41,6 +41,21 @@ Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since
 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.
 
 ```python
+def translate_participation(state: BeaconState, pending_attestations: Sequence[PendingAttestation]) -> None:
+    for attestation in pending_attestations:
+        data = attestation.data
+        inclusion_delay = attestation.inclusion_delay
+        # Translate attestation inclusion info to flag indices
+        participation_flag_indices = get_attestation_participation_flag_indices(state, data, inclusion_delay)
+
+        # Apply flags to all attesting validators
+        epoch_participation = state.previous_epoch_participation
+        for index in get_attesting_indices(state, data, attestation.aggregation_bits):
+            for flag_index, weight in get_flag_indices_and_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)
+
+
 def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
     epoch = phase0.get_current_epoch(pre)
     post = BeaconState(
@@ -80,6 +95,8 @@ def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
         # Inactivity
         inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
     )
+    # Fill in previous epoch participation from the pre state's pending attestations
+    translate_participation(post, pre.previous_epoch_attestations)
     # 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)