diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index 75190920e..8e26093a9 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -19,26 +19,6 @@ import
 
 export extras, phase0, altair, merge
 
-# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/beacon-chain.md#is_valid_merkle_branch
-func is_valid_merkle_branch*(leaf: Eth2Digest, branch: openArray[Eth2Digest],
-                             depth: int, index: uint64,
-                             root: Eth2Digest): bool {.nbench.}=
-  ## Check if ``leaf`` at ``index`` verifies against the Merkle ``root`` and
-  ## ``branch``.
-  var
-    value = leaf
-    buf: array[64, byte]
-
-  for i in 0 ..< depth:
-    if (index div (1'u64 shl i)) mod 2 != 0:
-      buf[0..31] = branch[i].data
-      buf[32..63] = value.data
-    else:
-      buf[0..31] = value.data
-      buf[32..63] = branch[i].data
-    value = eth2digest(buf)
-  value == root
-
 # https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/beacon-chain.md#increase_balance
 func increase_balance*(balance: var Gwei, delta: Gwei) =
   balance += delta
diff --git a/beacon_chain/spec/datatypes/altair.nim b/beacon_chain/spec/datatypes/altair.nim
index 738128b89..b76fd4a13 100644
--- a/beacon_chain/spec/datatypes/altair.nim
+++ b/beacon_chain/spec/datatypes/altair.nim
@@ -28,7 +28,7 @@
 #  stew/byteutils,
 
 import
-  std/[macros, typetraits],
+  std/[macros, typetraits, sets, hashes],
   chronicles,
   stew/[assign2, bitops2],
   json_serialization/types as jsonTypes
@@ -174,6 +174,12 @@ type
     fork_version*: Version ##\
     ## Fork version for the aggregate signature
 
+  # https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.4/specs/altair/sync-protocol.md#lightclientstore
+  LightClientStore* = object
+    snapshot*: LightClientSnapshot
+    valid_updates*: HashSet[LightClientUpdate]
+      ## TODO: This will benefit from being an ordered set
+
   # https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.4/specs/altair/beacon-chain.md#beaconstate
   BeaconState* = object
     # Versioning
@@ -518,3 +524,7 @@ func shortLog*(v: SyncAggregate): auto =
   $(v.sync_committee_bits)
 
 chronicles.formatIt SyncCommitteeMessage: shortLog(it)
+
+func hash*(x: LightClientUpdate): Hash =
+  hash(x.header.state_root.data)
+
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 1b404864a..7ab91304b 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -13,7 +13,7 @@ import
   # Standard lib
   std/[math, tables],
   # Third-party
-  stew/[byteutils, endians2],
+  stew/[byteutils, endians2, bitops2],
   # Internal
   ./datatypes/[phase0, altair, merge],
   ./eth2_merkleization, ./ssz_codec
@@ -47,6 +47,26 @@ template epoch*(slot: Slot): Epoch =
 template isEpoch*(slot: Slot): bool =
   (slot mod SLOTS_PER_EPOCH) == 0
 
+# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/beacon-chain.md#is_valid_merkle_branch
+func is_valid_merkle_branch*(leaf: Eth2Digest, branch: openArray[Eth2Digest],
+                             depth: int, index: uint64,
+                             root: Eth2Digest): bool =
+  ## Check if ``leaf`` at ``index`` verifies against the Merkle ``root`` and
+  ## ``branch``.
+  var
+    value = leaf
+    buf: array[64, byte]
+
+  for i in 0 ..< depth:
+    if (index div (1'u64 shl i)) mod 2 != 0:
+      buf[0..31] = branch[i].data
+      buf[32..63] = value.data
+    else:
+      buf[0..31] = value.data
+      buf[32..63] = branch[i].data
+    value = eth2digest(buf)
+  value == root
+
 const SLOTS_PER_SYNC_COMMITTEE_PERIOD* =
   EPOCHS_PER_SYNC_COMMITTEE_PERIOD * SLOTS_PER_EPOCH
 
@@ -213,3 +233,8 @@ func add_flag*(flags: ParticipationFlags, flag_index: int): ParticipationFlags =
 func has_flag*(flags: ParticipationFlags, flag_index: int): bool =
   let flag = ParticipationFlags(1'u8 shl flag_index)
   (flags and flag) == flag
+
+# https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#get_subtree_index
+func get_subtree_index*(idx: GeneralizedIndex): uint64 =
+  uint64(idx mod (type(idx)(1) shl log2trunc(idx)))
+
diff --git a/beacon_chain/spec/light_client_sync.nim b/beacon_chain/spec/light_client_sync.nim
new file mode 100644
index 000000000..3544752e1
--- /dev/null
+++ b/beacon_chain/spec/light_client_sync.nim
@@ -0,0 +1,113 @@
+import
+  std/sets,
+  stew/bitops2,
+  datatypes/altair,
+  helpers
+
+func branchIsAllZeros(branch: openarray[Eth2Digest]): bool =
+  for node in branch:
+    if node != Eth2Digest():
+      return false
+
+  return true
+
+# https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#validate_light_client_update
+proc validate_light_client_update*(snapshot: LightClientSnapshot,
+                                   update: LightClientUpdate,
+                                   genesis_validators_root: Eth2Digest): bool =
+  # Verify update slot is larger than snapshot slot
+  if update.header.slot <= snapshot.header.slot:
+    return false
+
+  # Verify update does not skip a sync committee period
+  var snapshot_period = compute_epoch_at_slot(snapshot.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+  var update_period = compute_epoch_at_slot(update.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+  if update_period notin [snapshot_period, snapshot_period + 1]:
+    return false
+
+  # Verify update header root is the finalized root of the finality header, if specified
+  # TODO: Use a view type instead of `unsafeAddr`
+  let signed_header = if update.finality_header == BeaconBlockHeader():
+    if not branchIsAllZeros(update.finality_branch):
+      return false
+    unsafeAddr update.header
+  else:
+    if not is_valid_merkle_branch(hash_tree_root(update.header),
+                                  update.finality_branch,
+                                  log2trunc(FINALIZED_ROOT_INDEX),
+                                  get_subtree_index(FINALIZED_ROOT_INDEX),
+                                  update.finality_header.state_root):
+      return false
+    unsafeAddr update.finality_header
+
+  # Verify update next sync committee if the update period incremented
+  # TODO: Use a view type instead of `unsafeAddr`
+  let sync_committee = if update_period == snapshot_period:
+    if not branchIsAllZeros(update.next_sync_committee_branch):
+      return false
+    unsafeAddr snapshot.current_sync_committee
+  else:
+    if not is_valid_merkle_branch(hash_tree_root(update.next_sync_committee),
+                                  update.next_sync_committee_branch,
+                                  log2trunc(NEXT_SYNC_COMMITTEE_INDEX),
+                                  get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
+                                  update.header.state_root):
+      return false
+    unsafeAddr snapshot.next_sync_committee
+
+  # Verify sync committee has sufficient participants
+  if countOnes(update.sync_committee_bits) < MIN_SYNC_COMMITTEE_PARTICIPANTS:
+    return false
+
+  # Verify sync committee aggregate signature
+  # participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
+  var participant_pubkeys: seq[ValidatorPubKey]
+  for idx, bit in update.sync_committee_bits:
+    if bit:
+      participant_pubkeys.add(sync_committee.pubkeys[idx])
+
+  let domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
+  let signing_root = compute_signing_root(signed_header[], domain)
+
+  blsFastAggregateVerify(participant_pubkeys, signing_root.data, update.sync_committee_signature)
+
+# https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#apply_light_client_update
+proc apply_light_client_update(snapshot: var LightClientSnapshot, update: LightClientUpdate) =
+  let snapshot_period = compute_epoch_at_slot(snapshot.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+  let update_period = compute_epoch_at_slot(update.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+  if update_period == snapshot_period + 1:
+      snapshot.current_sync_committee = snapshot.next_sync_committee
+      snapshot.next_sync_committee = update.next_sync_committee
+  snapshot.header = update.header
+
+# https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#process_light_client_update
+proc process_light_client_update(store: var LightClientStore,
+                                 update: LightClientUpdate,
+                                 current_slot: Slot,
+                                 genesis_validators_root: Eth2Digest): bool =
+  if not validate_light_client_update(store.snapshot, update, genesis_validators_root):
+    return false
+  store.valid_updates.incl(update)
+
+  var update_timeout = SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+  let sync_committee_participants_count = countOnes(update.sync_committee_bits)
+  if sync_committee_participants_count * 3 >= update.sync_committee_bits.len * 2 and
+     update.finality_header != BeaconBlockHeader():
+    # Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
+    # Note that (2) means that the current light client design needs finality.
+    # It may be changed to re-organizable light client design. See the on-going issue eth2.0-specs#2182.
+    apply_light_client_update(store.snapshot, update)
+    store.valid_updates.clear()
+  elif current_slot > store.snapshot.header.slot + update_timeout:
+    var best_update_participants = 0
+    var best_update: LightClientUpdate
+    for update in store.valid_updates:
+      let update_participants = countOnes(update.sync_committee_bits)
+      if update_participants > best_update_participants:
+        best_update = update
+        best_update_participants = update_participants
+
+    # Forced best update when the update timeout has elapsed
+    apply_light_client_update(store.snapshot, best_update)
+    store.valid_updates.clear()
+  return true
diff --git a/beacon_chain/spec/presets/mainnet/altair_preset.nim b/beacon_chain/spec/presets/mainnet/altair_preset.nim
index e2b4bb765..f53ec4871 100644
--- a/beacon_chain/spec/presets/mainnet/altair_preset.nim
+++ b/beacon_chain/spec/presets/mainnet/altair_preset.nim
@@ -22,4 +22,4 @@ const
   # Sync protocol
   # ---------------------------------------------------------------
   # 1
-  MIN_SYNC_COMMITTEE_PARTICIPANTS*: uint64 = 1
+  MIN_SYNC_COMMITTEE_PARTICIPANTS* = 1
diff --git a/beacon_chain/spec/presets/minimal/altair_preset.nim b/beacon_chain/spec/presets/minimal/altair_preset.nim
index ade62dcc9..e0bdc1a8d 100644
--- a/beacon_chain/spec/presets/minimal/altair_preset.nim
+++ b/beacon_chain/spec/presets/minimal/altair_preset.nim
@@ -22,4 +22,4 @@ const
   # Sync protocol
   # ---------------------------------------------------------------
   # 1
-  MIN_SYNC_COMMITTEE_PARTICIPANTS*: uint64 = 1
+  MIN_SYNC_COMMITTEE_PARTICIPANTS* = 1
diff --git a/beacon_chain/ssz/merkleization.nim b/beacon_chain/ssz/merkleization.nim
index eaaee18e5..1d4b6a9d9 100644
--- a/beacon_chain/ssz/merkleization.nim
+++ b/beacon_chain/ssz/merkleization.nim
@@ -32,6 +32,8 @@ func binaryTreeHeight*(totalElements: Limit): int =
   bitWidth nextPow2(uint64 totalElements)
 
 type
+  GeneralizedIndex* = uint32
+
   SszMerkleizerImpl = object
     combinedChunks: ptr UncheckedArray[Eth2Digest]
     totalChunks: uint64