diff --git a/beacon_chain/mainchain_monitor.nim b/beacon_chain/mainchain_monitor.nim
index 70e3fed9b..f2cec4182 100644
--- a/beacon_chain/mainchain_monitor.nim
+++ b/beacon_chain/mainchain_monitor.nim
@@ -91,14 +91,15 @@ const
 # module seems broken. Investigate and file this as an issue.
 {.push warning[LockLevel]: off.}
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#get_eth1_data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/validator.md#get_eth1_data
 func compute_time_at_slot(state: BeaconState, slot: Slot): uint64 =
-  return state.genesis_time + slot * SECONDS_PER_SLOT
+  state.genesis_time + slot * SECONDS_PER_SLOT
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#get_eth1_data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/validator.md#get_eth1_data
 func voting_period_start_time*(state: BeaconState): uint64 =
-  let eth1_voting_period_start_slot = state.slot - state.slot mod SLOTS_PER_ETH1_VOTING_PERIOD.uint64
-  return compute_time_at_slot(state, eth1_voting_period_start_slot)
+  let eth1_voting_period_start_slot =
+    state.slot - state.slot mod SLOTS_PER_ETH1_VOTING_PERIOD.uint64
+  compute_time_at_slot(state, eth1_voting_period_start_slot)
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#get_eth1_data
 func is_candidate_block(blk: Eth1Block, period_start: uint64): bool =
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index ded59fc2a..be492e861 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -98,7 +98,7 @@ const
     # TODO: This needs revisiting.
     # Why was the validator WITHDRAWAL_PERIOD altered in the spec?
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.2/specs/phase0/p2p-interface.md#configuration
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/p2p-interface.md#configuration
   ATTESTATION_PROPAGATION_SLOT_RANGE* = 32
 
   SLOTS_PER_ETH1_VOTING_PERIOD* = Slot(EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH)
@@ -242,7 +242,7 @@ type
 
     body*: BeaconBlockBody
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#beaconblockheader
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#beaconblockheader
   BeaconBlockHeader* = object
     slot*: Slot
     proposer_index*: uint64
@@ -263,7 +263,7 @@ type
     deposits*: List[Deposit, MAX_DEPOSITS]
     voluntary_exits*: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#beaconstate
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#beaconstate
   BeaconStateObj* = object
     # Versioning
     genesis_time*: uint64
@@ -357,7 +357,7 @@ type
     block_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
     state_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#fork
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#fork
   Fork* = object
     # TODO: Spec introduced an alias for Version = array[4, byte]
     #       and a default parameter to compute_domain
diff --git a/beacon_chain/spec/state_transition_block.nim b/beacon_chain/spec/state_transition_block.nim
index 98ba0ddcd..184f41080 100644
--- a/beacon_chain/spec/state_transition_block.nim
+++ b/beacon_chain/spec/state_transition_block.nim
@@ -41,7 +41,7 @@ declareGauge beacon_previous_live_validators, "Number of active validators that
 declareGauge beacon_pending_deposits, "Number of pending deposits (state.eth1_data.deposit_count - state.eth1_deposit_index)" # On block
 declareGauge beacon_processed_deposits_total, "Number of total deposits included on chain" # On block
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#block-header
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#block-header
 proc process_block_header*(
     state: var BeaconState, blck: BeaconBlock, flags: UpdateFlags,
     stateCache: var StateCache): bool {.nbench.}=
@@ -405,7 +405,7 @@ func get_slot_signature*(
 
   blsSign(privKey, signing_root.data)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#randao-reveal
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/validator.md#randao-reveal
 func get_epoch_signature*(
     fork: Fork, genesis_validators_root: Eth2Digest, slot: Slot,
     privkey: ValidatorPrivKey): ValidatorSig =
@@ -416,7 +416,7 @@ func get_epoch_signature*(
 
   blsSign(privKey, signing_root.data)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#signature
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/validator.md#signature
 func get_block_signature*(
     fork: Fork, genesis_validators_root: Eth2Digest, slot: Slot,
     root: Eth2Digest, privkey: ValidatorPrivKey): ValidatorSig =
@@ -440,7 +440,7 @@ func get_aggregate_and_proof_signature*(fork: Fork, genesis_validators_root: Eth
 
   return blsSign(privKey, signing_root.data)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#aggregate-signature
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/validator.md#aggregate-signature
 func get_attestation_signature*(
     fork: Fork, genesis_validators_root: Eth2Digest, attestation: AttestationData,
     privkey: ValidatorPrivKey): ValidatorSig =
diff --git a/beacon_chain/spec/state_transition_epoch.nim b/beacon_chain/spec/state_transition_epoch.nim
index 0fc63f182..d7b50b196 100644
--- a/beacon_chain/spec/state_transition_epoch.nim
+++ b/beacon_chain/spec/state_transition_epoch.nim
@@ -539,7 +539,7 @@ func process_slashings*(state: var BeaconState, cache: var StateCache) {.nbench.
       let penalty = penalty_numerator div total_balance * increment
       decrease_balance(state, index.ValidatorIndex, penalty)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#final-updates
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#final-updates
 func process_final_updates*(state: var BeaconState) {.nbench.}=
   let
     current_epoch = get_current_epoch(state)
diff --git a/beacon_chain/spec/state_transition_helpers.nim b/beacon_chain/spec/state_transition_helpers.nim
index 12d070abd..5e3935d00 100644
--- a/beacon_chain/spec/state_transition_helpers.nim
+++ b/beacon_chain/spec/state_transition_helpers.nim
@@ -35,7 +35,7 @@ func get_attesting_indices*(
     result = result.union(get_attesting_indices(
       state, a.data, a.aggregation_bits, stateCache))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#helper-functions-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#helper-functions-1
 func get_unslashed_attesting_indices*(
     state: BeaconState, attestations: openarray[PendingAttestation],
     stateCache: var StateCache): HashSet[ValidatorIndex] =