diff --git a/specs/_features/eip6914/beacon-chain.md b/specs/_features/eip6914/beacon-chain.md
new file mode 100644
index 000000000..2c60c9bdb
--- /dev/null
+++ b/specs/_features/eip6914/beacon-chain.md
@@ -0,0 +1,65 @@
+EIP-6914 -- The Beacon Chain
+
+## Table of contents
+
+<!-- TOC -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Preset](#preset)
+  - [Time parameters](#time-parameters)
+- [Helper functions](#helper-functions)
+  - [Predicates](#predicates)
+    - [`is_reusable_validator`](#is_reusable_validator)
+- [Beacon chain state transition function](#beacon-chain-state-transition-function)
+  - [Block processing](#block-processing)
+    - [Modified `get_index_for_new_validator`](#modified-get_index_for_new_validator)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the beacon chain specification to assign new deposits to existing validator records. Refers to [EIP-6914](https://github.com/ethereum/EIPs/pull/6914).
+
+*Note:* This specification is built upon [Capella](../../capella/beacon_chain.md) and is under active development.
+
+## Preset
+
+### Time parameters
+
+| Name | Value | Unit | Duration |
+| - | - | - | - |
+| `SAFE_EPOCHS_TO_REUSE_INDEX` | `uint64(2**16)` (= 65,536) | epochs | ~0.8 year |
+
+## Helper functions
+
+### Predicates
+
+#### `is_reusable_validator`
+
+```python
+def is_reusable_validator(validator: Validator, balance: Gwei, epoch: Epoch) -> bool:
+    """
+    Check if ``validator`` index can be re-assigned to a new deposit.
+    """
+    return (
+        epoch > validator.withdrawable_epoch + SAFE_EPOCHS_TO_REUSE_INDEX
+        and balance == 0
+    )
+```
+
+## Beacon chain state transition function
+
+### Block processing
+
+#### Modified `get_index_for_new_validator`
+
+```python
+def get_index_for_new_validator(state: BeaconState) -> ValidatorIndex:
+    for index, validator in enumerate(state.validators):
+        if is_reusable_validator(validator, state.balances[index], get_current_epoch(state)):
+            return ValidatorIndex(index)
+    return ValidatorIndex(len(state.validators))
+```
diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 58dfad608..8c3a8877e 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -30,6 +30,8 @@
   - [Misc](#misc-1)
     - [`add_flag`](#add_flag)
     - [`has_flag`](#has_flag)
+    - [`get_index_for_new_validator`](#get_index_for_new_validator)
+    - [`set_or_append_list`](#set_or_append_list)
   - [Beacon state accessors](#beacon-state-accessors)
     - [`get_next_sync_committee_indices`](#get_next_sync_committee_indices)
     - [`get_next_sync_committee`](#get_next_sync_committee)
@@ -248,6 +250,23 @@ def has_flag(flags: ParticipationFlags, flag_index: int) -> bool:
     return flags & flag == flag
 ```
 
+#### `get_index_for_new_validator`
+
+```python
+def get_index_for_new_validator(state: BeaconState) -> ValidatorIndex:
+    return ValidatorIndex(len(state.validators))
+```
+
+#### `set_or_append_list`
+
+```python
+def set_or_append_list(list: List, index: ValidatorIndex, value: Any) -> None:
+    if index == len(list):
+        list.append(value)
+    else:
+        list[index] = value
+```
+
 ### Beacon state accessors
 
 #### `get_next_sync_committee_indices`
@@ -511,12 +530,14 @@ def apply_deposit(state: BeaconState,
         signing_root = compute_signing_root(deposit_message, domain)
         # Initialize validator if the deposit signature is valid
         if bls.Verify(pubkey, signing_root, signature):
-            state.validators.append(get_validator_from_deposit(pubkey, withdrawal_credentials, amount))
-            state.balances.append(amount)
+            index = get_index_for_new_validator(state)
+            validator = get_validator_from_deposit(pubkey, withdrawal_credentials, amount)
+            set_or_append_list(state.validators, index, validator)
+            set_or_append_list(state.balances, index, amount)
             # [New in Altair]
-            state.previous_epoch_participation.append(ParticipationFlags(0b0000_0000))
-            state.current_epoch_participation.append(ParticipationFlags(0b0000_0000))
-            state.inactivity_scores.append(uint64(0))
+            set_or_append_list(state.previous_epoch_participation, index, ParticipationFlags(0b0000_0000))
+            set_or_append_list(state.current_epoch_participation, index, ParticipationFlags(0b0000_0000))
+            set_or_append_list(state.inactivity_scores, index, uint64(0))
     else:
         # Increase balance by deposit amount
         index = ValidatorIndex(validator_pubkeys.index(pubkey))