diff --git a/beacon_chain/interop.nim b/beacon_chain/interop.nim
index ff6de75c3..16c3e920b 100644
--- a/beacon_chain/interop.nim
+++ b/beacon_chain/interop.nim
@@ -41,7 +41,7 @@ const eth1BlockHash* = block:
   x
 
 func makeWithdrawalCredentials*(k: ValidatorPubKey): Eth2Digest =
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_deposit-contract.md#withdrawal-credentials
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_deposit-contract.md#withdrawal-credentials
   var bytes = eth2hash(k.getBytes())
   bytes.data[0] = BLS_WITHDRAWAL_PREFIX.uint8
   bytes
diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index c6d546690..fecdf16ae 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -11,7 +11,7 @@ import
   ../extras, ../ssz,
   ./crypto, ./datatypes, ./digest, ./helpers, ./validator
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_valid_merkle_branch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#is_valid_merkle_branch
 func is_valid_merkle_branch(leaf: Eth2Digest, branch: openarray[Eth2Digest], depth: uint64, index: uint64, root: Eth2Digest): bool =
   ## Check if ``leaf`` at ``index`` verifies against the Merkle ``root`` and
   ## ``branch``.
@@ -29,13 +29,13 @@ func is_valid_merkle_branch(leaf: Eth2Digest, branch: openarray[Eth2Digest], dep
     value = eth2hash(buf)
   value == root
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#increase_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#increase_balance
 func increase_balance*(
     state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
   # Increase the validator balance at index ``index`` by ``delta``.
   state.balances[index] += delta
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#decrease_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#decrease_balance
 func decrease_balance*(
     state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
   ## Decrease the validator balance at index ``index`` by ``delta``, with
@@ -102,13 +102,13 @@ func process_deposit*(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_activation_exit_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_activation_exit_epoch
 func compute_activation_exit_epoch*(epoch: Epoch): Epoch =
   ## Return the epoch during which validator activations and exits initiated in
   ## ``epoch`` take effect.
   epoch + 1 + MAX_SEED_LOOKAHEAD
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_validator_churn_limit
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_validator_churn_limit
 func get_validator_churn_limit(state: BeaconState): uint64 =
   # Return the validator churn limit for the current epoch.
   let active_validator_indices =
@@ -116,7 +116,7 @@ func get_validator_churn_limit(state: BeaconState): uint64 =
   max(MIN_PER_EPOCH_CHURN_LIMIT,
     len(active_validator_indices) div CHURN_LIMIT_QUOTIENT).uint64
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#initiate_validator_exit
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#initiate_validator_exit
 func initiate_validator_exit*(state: var BeaconState,
                               index: ValidatorIndex) =
   # Initiate the exit of the validator with index ``index``.
@@ -146,7 +146,7 @@ func initiate_validator_exit*(state: var BeaconState,
   validator.withdrawable_epoch =
     validator.exit_epoch + MIN_VALIDATOR_WITHDRAWABILITY_DELAY
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#slash_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#slash_validator
 func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
     stateCache: var StateCache) =
   # Slash the validator with index ``index``.
@@ -306,7 +306,7 @@ func get_attestation_data_slot*(state: BeaconState,
     state, data, get_committee_count_at_slot(
       state, data.target.epoch.compute_start_slot_at_epoch) * SLOTS_PER_EPOCH)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_block_root_at_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_block_root_at_slot
 func get_block_root_at_slot*(state: BeaconState,
                              slot: Slot): Eth2Digest =
   # Return the block root at a recent ``slot``.
@@ -315,12 +315,12 @@ func get_block_root_at_slot*(state: BeaconState,
   doAssert slot < state.slot
   state.block_roots[slot mod SLOTS_PER_HISTORICAL_ROOT]
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_block_root
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_block_root
 func get_block_root*(state: BeaconState, epoch: Epoch): Eth2Digest =
   # Return the block root at the start of a recent ``epoch``.
   get_block_root_at_slot(state, compute_start_slot_at_epoch(epoch))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_total_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_total_balance
 func get_total_balance*(state: BeaconState, validators: auto): Gwei =
   ## Return the combined effective balance of the ``indices``. (1 Gwei minimum
   ## to avoid divisions by zero.)
@@ -329,7 +329,7 @@ func get_total_balance*(state: BeaconState, validators: auto): Gwei =
   )
 
 # XXX: Move to state_transition_epoch.nim?
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#registry-updates
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#registry-updates
 func process_registry_updates*(state: var BeaconState) =
   ## Process activation eligibility and ejections
   ## Try to avoid caching here, since this could easily become undefined
@@ -430,7 +430,7 @@ proc is_valid_indexed_attestation*(
   if not result:
     notice "indexed attestation: signature verification failure"
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_attesting_indices
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_attesting_indices
 func get_attesting_indices*(state: BeaconState,
                             data: AttestationData,
                             bits: CommitteeValidatorsBits,
@@ -451,7 +451,7 @@ func get_attesting_indices_seq*(state: BeaconState,
   toSeq(items(get_attesting_indices(
     state, attestation_data, bits, cache)))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_indexed_attestation
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_indexed_attestation
 func get_indexed_attestation*(state: BeaconState, attestation: Attestation,
     stateCache: var StateCache): IndexedAttestation =
   # Return the indexed attestation corresponding to ``attestation``.
diff --git a/beacon_chain/spec/crypto.nim b/beacon_chain/spec/crypto.nim
index 86f9f3a21..ac799457b 100644
--- a/beacon_chain/spec/crypto.nim
+++ b/beacon_chain/spec/crypto.nim
@@ -162,11 +162,11 @@ proc combine*[T](x: var BlsValue[T], other: BlsValue[T]) =
   doAssert x.kind == Real and other.kind == Real
   x.blsValue.combine(other.blsValue)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/bls_signature.md#bls_aggregate_pubkeys
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/bls_signature.md#bls_aggregate_pubkeys
 func bls_aggregate_pubkeys*(keys: openArray[ValidatorPubKey]): ValidatorPubKey =
   keys.combine()
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/bls_signature.md#bls_verify
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/bls_signature.md#bls_verify
 func bls_verify*(
     pubkey: ValidatorPubKey, msg: openArray[byte], sig: ValidatorSig,
     domain: Domain): bool =
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index 06a40049f..911f8706f 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -59,7 +59,7 @@ const
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#initial-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#initial-values
   GENESIS_EPOCH* = (GENESIS_SLOT.uint64 div SLOTS_PER_EPOCH).Epoch ##\
   ## compute_epoch_at_slot(GENESIS_SLOT)
 
@@ -86,13 +86,13 @@ type
 
   BitList*[maxLen: static int] = distinct BitSeq
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#proposerslashing
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#proposerslashing
   ProposerSlashing* = object
     proposer_index*: uint64
     header_1*: BeaconBlockHeader
     header_2*: BeaconBlockHeader
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#attesterslashing
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#attesterslashing
   AttesterSlashing* = object
     attestation_1*: IndexedAttestation
     attestation_2*: IndexedAttestation
@@ -119,12 +119,12 @@ type
     custody_bits*: CommitteeValidatorsBits
     signature*: ValidatorSig
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#checkpoint
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#checkpoint
   Checkpoint* = object
     epoch*: Epoch
     root*: Eth2Digest
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#AttestationData
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#AttestationData
   AttestationData* = object
     slot*: Slot
     index*: uint64
@@ -143,21 +143,21 @@ type
     custody_bit*: bool ##\
     ## Challengeable bit (SSZ-bool, 1 byte) for the custody of shard data
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#deposit
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#deposit
   Deposit* = object
     proof*: array[DEPOSIT_CONTRACT_TREE_DEPTH + 1, Eth2Digest] ##\
     ## Merkle path to deposit data list root
 
     data*: DepositData
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#depositdata
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#depositdata
   DepositData* = object
     pubkey*: ValidatorPubKey
     withdrawal_credentials*: Eth2Digest
     amount*: uint64
     signature*: ValidatorSig
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#voluntaryexit
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#voluntaryexit
   VoluntaryExit* = object
     epoch*: Epoch ##\
     ## Earliest epoch when voluntary exit can be processed
@@ -165,7 +165,7 @@ type
     validator_index*: uint64
     signature*: ValidatorSig
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beaconblock
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beaconblock
   BeaconBlock* = object
     ## For each slot, a proposer is chosen from the validator pool to propose
     ## a new block. Once the block as been proposed, it is transmitted to
@@ -186,7 +186,7 @@ type
     signature*: ValidatorSig ##\
     ## Proposer signature
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beaconblockheader
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beaconblockheader
   BeaconBlockHeader* = object
     slot*: Slot
     parent_root*: Eth2Digest
@@ -194,7 +194,7 @@ type
     body_root*: Eth2Digest
     signature*: ValidatorSig
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beaconblockbody
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beaconblockbody
   BeaconBlockBody* = object
     randao_reveal*: ValidatorSig
     eth1_data*: Eth1Data
@@ -207,7 +207,7 @@ type
     deposits*: seq[Deposit]
     voluntary_exits*: seq[VoluntaryExit]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beaconstate
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beaconstate
   BeaconStateNew* = object
     # Versioning
     genesis_time*: uint64
@@ -265,7 +265,7 @@ type
     current_justified_checkpoint*: Checkpoint
     finalized_checkpoint*: Checkpoint
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beaconstate
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beaconstate
   BeaconState* = object
     # Versioning
     genesis_time*: uint64
@@ -324,7 +324,7 @@ type
     current_justified_checkpoint*: Checkpoint
     finalized_checkpoint*: Checkpoint
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#validator
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#validator
   Validator* = object
     pubkey*: ValidatorPubKey
 
@@ -347,7 +347,7 @@ type
     withdrawable_epoch*: Epoch ##\
     ## When validator can withdraw or transfer funds
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#pendingattestation
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#pendingattestation
   PendingAttestation* = object
     aggregation_bits*: CommitteeValidatorsBits
     data*: AttestationData
@@ -357,12 +357,12 @@ type
 
     proposer_index*: uint64
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#historicalbatch
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#historicalbatch
   HistoricalBatch* = object
     block_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
     state_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#fork
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#fork
   Fork* = object
     # TODO: Spec introduced an alias for Version = array[4, byte]
     #       and a default parameter to compute_domain
@@ -372,7 +372,7 @@ type
     epoch*: Epoch ##\
     ## Epoch of latest fork
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#eth1data
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#eth1data
   Eth1Data* = object
     deposit_root*: Eth2Digest
     deposit_count*: uint64
diff --git a/beacon_chain/spec/digest.nim b/beacon_chain/spec/digest.nim
index a2d647347..6471e6e54 100644
--- a/beacon_chain/spec/digest.nim
+++ b/beacon_chain/spec/digest.nim
@@ -7,7 +7,7 @@
 
 # Serenity hash function / digest
 #
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#hash
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#hash
 #
 # In Phase 0 the beacon chain is deployed with SHA256 (SHA2-256).
 # Note that is is different from Keccak256 (often mistakenly called SHA3-256)
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 75831372f..ca7376d67 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -15,7 +15,7 @@ import
   # Internal
   ./datatypes, ./digest
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#integer_squareroot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#integer_squareroot
 func integer_squareroot*(n: SomeInteger): SomeInteger =
   # Return the largest integer ``x`` such that ``x**2 <= n``.
   doAssert n >= 0'u64
@@ -28,7 +28,7 @@ func integer_squareroot*(n: SomeInteger): SomeInteger =
     y = (x + n div x) div 2
   x
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_epoch_at_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_epoch_at_slot
 func compute_epoch_at_slot*(slot: Slot|uint64): Epoch =
   # Return the epoch number of the given ``slot``.
   (slot div SLOTS_PER_EPOCH).Epoch
@@ -36,17 +36,17 @@ func compute_epoch_at_slot*(slot: Slot|uint64): Epoch =
 template epoch*(slot: Slot): Epoch =
   compute_epoch_at_slot(slot)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_start_slot_at_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_start_slot_at_epoch
 func compute_start_slot_at_epoch*(epoch: Epoch): Slot =
   # Return the start slot of ``epoch``.
   (epoch * SLOTS_PER_EPOCH).Slot
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_active_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#is_active_validator
 func is_active_validator*(validator: Validator, epoch: Epoch): bool =
   ### Check if ``validator`` is active
   validator.activation_epoch <= epoch and epoch < validator.exit_epoch
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_active_validator_indices
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_active_validator_indices
 func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
     seq[ValidatorIndex] =
   # Return the sequence of active validator indices at ``epoch``.
@@ -54,7 +54,7 @@ func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
     if is_active_validator(val, epoch):
       result.add idx.ValidatorIndex
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_committee_count_at_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_committee_count_at_slot
 func get_committee_count_at_slot*(state: BeaconState, slot: Slot): uint64 =
   # Return the number of committees at ``slot``.
   let epoch = compute_epoch_at_slot(slot)
@@ -68,13 +68,13 @@ func get_committee_count_at_slot*(state: BeaconState, slot: Slot): uint64 =
   # committees.
   doAssert (SLOTS_PER_EPOCH * MAX_COMMITTEES_PER_SLOT).uint64 >= result
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_current_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_current_epoch
 func get_current_epoch*(state: BeaconState): Epoch =
   # Return the current epoch.
   doAssert state.slot >= GENESIS_SLOT, $state.slot
   compute_epoch_at_slot(state.slot)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_randao_mix
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_randao_mix
 func get_randao_mix*(state: BeaconState,
                      epoch: Epoch): Eth2Digest =
     ## Returns the randao mix at a recent ``epoch``.
@@ -115,14 +115,14 @@ func int_to_bytes4*(x: uint64): array[4, byte] =
   result[2] = ((x shr 16) and 0xff).byte
   result[3] = ((x shr 24) and 0xff).byte
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_domain
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_domain
 func compute_domain*(
     domain_type: DomainType,
     fork_version: array[4, byte] = [0'u8, 0, 0, 0]): Domain =
   result[0..3] = int_to_bytes4(domain_type.uint64)
   result[4..7] = fork_version
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_domain
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_domain
 func get_domain*(
     state: BeaconState, domain_type: DomainType, message_epoch: Epoch): Domain =
   ## Return the signature domain (fork version concatenated with domain type)
@@ -138,7 +138,7 @@ func get_domain*(
 func get_domain*(state: BeaconState, domain_type: DomainType): Domain =
   get_domain(state, domain_type, get_current_epoch(state))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_seed
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_seed
 func get_seed*(state: BeaconState, epoch: Epoch, domain_type: DomainType): Eth2Digest =
   # Generate a seed for the given ``epoch``.
 
diff --git a/beacon_chain/spec/presets/mainnet.nim b/beacon_chain/spec/presets/mainnet.nim
index 32fbba5eb..bde340728 100644
--- a/beacon_chain/spec/presets/mainnet.nim
+++ b/beacon_chain/spec/presets/mainnet.nim
@@ -49,14 +49,14 @@ const
   SHUFFLE_ROUND_COUNT* = 90
 
   # Constants (TODO: not actually configurable)
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#constants
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#constants
   BASE_REWARDS_PER_EPOCH* = 4
 
   DEPOSIT_CONTRACT_TREE_DEPTH* = 32
 
   # Gwei values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/mainnet.yaml#L33
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/mainnet.yaml#L33
 
   MIN_DEPOSIT_AMOUNT* = 2'u64^0 * 10'u64^9 ##\
   ## Minimum amounth of ETH that can be deposited in one call - deposits can
@@ -73,14 +73,14 @@ const
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/configs/mainnet.yaml#L45
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.4/configs/mainnet.yaml#L45
 
   GENESIS_SLOT* = 0.Slot
   BLS_WITHDRAWAL_PREFIX* = 0'u8
 
   # Time parameters
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/configs/mainnet.yaml#L52
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.4/configs/mainnet.yaml#L52
 
   SECONDS_PER_SLOT*{.intdefine.} = 12'u64 # Compile with -d:SECONDS_PER_SLOT=1 for 6x faster slots
   ## TODO consistent time unit across projects, similar to C++ chrono?
@@ -131,7 +131,7 @@ const
 
   # State vector lengths
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/mainnet.yaml#L81
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/mainnet.yaml#L81
   EPOCHS_PER_HISTORICAL_VECTOR* = 65536
   EPOCHS_PER_SLASHINGS_VECTOR* = 8192
   HISTORICAL_ROOTS_LIMIT* = 16777216
@@ -139,7 +139,7 @@ const
 
   # Reward and penalty quotients
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/mainnet.yaml#L93
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/mainnet.yaml#L93
   BASE_REWARD_FACTOR* = 2'u64^6
   WHISTLEBLOWER_REWARD_QUOTIENT* = 2'u64^9
   PROPOSER_REWARD_QUOTIENT* = 2'u64^3
@@ -148,7 +148,7 @@ const
 
   # Max operations per block
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/mainnet.yaml#L107
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/mainnet.yaml#L107
   MAX_PROPOSER_SLASHINGS* = 2^4
   MAX_ATTESTER_SLASHINGS* = 2^0
   MAX_ATTESTATIONS* = 2^7
@@ -161,7 +161,7 @@ const
 type
   # Domains
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#domain-types
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#domain-types
   DomainType* {.pure.} = enum
     DOMAIN_BEACON_PROPOSER = 0
     DOMAIN_BEACON_ATTESTER = 1
diff --git a/beacon_chain/spec/presets/minimal.nim b/beacon_chain/spec/presets/minimal.nim
index be0e2d993..c9b18479b 100644
--- a/beacon_chain/spec/presets/minimal.nim
+++ b/beacon_chain/spec/presets/minimal.nim
@@ -42,7 +42,7 @@ const
 
   # Constants
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#constants
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#constants
   # TODO "The following values are (non-configurable) constants" ...
   # Unchanged
   BASE_REWARDS_PER_EPOCH* = 4
@@ -51,7 +51,7 @@ const
 
   # Gwei values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/minimal.yaml#L32
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/minimal.yaml#L32
 
   # Unchanged
   MIN_DEPOSIT_AMOUNT* = 2'u64^0 * 10'u64^9
@@ -61,7 +61,7 @@ const
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/minimal.yaml#L44
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/minimal.yaml#L44
 
   # Unchanged
   GENESIS_SLOT* = 0.Slot
@@ -95,7 +95,7 @@ const
 
   # State vector lengths
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/minimal.yaml#L83
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/minimal.yaml#L83
 
   # Changed
   EPOCHS_PER_HISTORICAL_VECTOR* = 64
@@ -105,7 +105,7 @@ const
 
   # Reward and penalty quotients
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/minimal.yaml#L95
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/minimal.yaml#L95
 
   BASE_REWARD_FACTOR* = 2'u64^6
   WHISTLEBLOWER_REWARD_QUOTIENT* = 2'u64^9
@@ -115,7 +115,7 @@ const
 
   # Max operations per block
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/configs/minimal.yaml#L109
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/configs/minimal.yaml#L109
 
   MAX_PROPOSER_SLASHINGS* = 2^4
   MAX_ATTESTER_SLASHINGS* = 2^0
@@ -127,7 +127,7 @@ const
 type
   # Domains
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#domain-types
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#domain-types
   DomainType* {.pure.} = enum
     DOMAIN_BEACON_PROPOSER = 0
     DOMAIN_BEACON_ATTESTER = 1
diff --git a/beacon_chain/spec/state_transition_block.nim b/beacon_chain/spec/state_transition_block.nim
index 7dc40b49f..ce09b397a 100644
--- a/beacon_chain/spec/state_transition_block.nim
+++ b/beacon_chain/spec/state_transition_block.nim
@@ -43,7 +43,7 @@ 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/v0.9.0/specs/core/0_beacon-chain.md#block-header
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#block-header
 proc process_block_header*(
     state: var BeaconState, blck: BeaconBlock, flags: UpdateFlags,
     stateCache: var StateCache): bool =
@@ -98,7 +98,7 @@ proc process_block_header*(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#randao
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#randao
 proc process_randao(
     state: var BeaconState, body: BeaconBlockBody, flags: UpdateFlags,
     stateCache: var StateCache): bool =
@@ -131,21 +131,21 @@ proc process_randao(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#eth1-data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#eth1-data
 func process_eth1_data(state: var BeaconState, body: BeaconBlockBody) =
   state.eth1_data_votes.add body.eth1_data
   if state.eth1_data_votes.count(body.eth1_data) * 2 >
       SLOTS_PER_ETH1_VOTING_PERIOD:
     state.eth1_data = body.eth1_data
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_slashable_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#is_slashable_validator
 func is_slashable_validator(validator: Validator, epoch: Epoch): bool =
   # Check if ``validator`` is slashable.
   (not validator.slashed) and
     (validator.activation_epoch <= epoch) and
     (epoch < validator.withdrawable_epoch)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#proposer-slashings
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#proposer-slashings
 proc process_proposer_slashing*(
     state: var BeaconState, proposer_slashing: ProposerSlashing,
     flags: UpdateFlags, stateCache: var StateCache): bool =
@@ -204,7 +204,7 @@ proc processProposerSlashings(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_slashable_attestation_data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#is_slashable_attestation_data
 func is_slashable_attestation_data(
     data_1: AttestationData, data_2: AttestationData): bool =
   ## Check if ``data_1`` and ``data_2`` are slashable according to Casper FFG
@@ -257,7 +257,7 @@ proc process_attester_slashing*(
       return false
     return true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#attester-slashings
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#attester-slashings
 proc processAttesterSlashings(state: var BeaconState, blck: BeaconBlock,
     stateCache: var StateCache): bool =
   # Process ``AttesterSlashing`` operation.
@@ -305,7 +305,7 @@ proc processDeposits(state: var BeaconState, blck: BeaconBlock): bool =
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#voluntary-exits
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#voluntary-exits
 proc process_voluntary_exit*(
     state: var BeaconState,
     exit: VoluntaryExit,
diff --git a/beacon_chain/spec/state_transition_epoch.nim b/beacon_chain/spec/state_transition_epoch.nim
index 6ea9eee3b..3e71133d4 100644
--- a/beacon_chain/spec/state_transition_epoch.nim
+++ b/beacon_chain/spec/state_transition_epoch.nim
@@ -55,14 +55,14 @@ declareGauge beacon_previous_justified_root, "Current previously justified root"
 # Spec
 # --------------------------------------------------------
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_total_active_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_total_active_balance
 func get_total_active_balance*(state: BeaconState): Gwei =
   # Return the combined effective balance of the active validators.
   return get_total_balance(
     state,
     get_active_validator_indices(state, get_current_epoch(state)))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#helper-functions-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#helper-functions-1
 func get_matching_source_attestations(state: BeaconState, epoch: Epoch):
     seq[PendingAttestation] =
   doAssert epoch in [get_current_epoch(state), get_previous_epoch(state)]
@@ -92,7 +92,7 @@ func get_attesting_balance(
   get_total_balance(state, get_unslashed_attesting_indices(
     state, attestations, stateCache))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#justification-and-finalization
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#justification-and-finalization
 proc process_justification_and_finalization*(
     state: var BeaconState, stateCache: var StateCache) =
 
@@ -131,11 +131,11 @@ proc process_justification_and_finalization*(
   ## matter -- in the next epoch, they'll be 2 epochs old, when BeaconState
   ## tracks current_epoch_attestations and previous_epoch_attestations only
   ## per
-  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#attestations
+  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#attestations
   ## and `get_matching_source_attestations(...)` via
-  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#helper-functions-1
+  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#helper-functions-1
   ## and
-  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#final-updates
+  ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#final-updates
   ## after which the state.previous_epoch_attestations is replaced.
   trace "Non-attesting indices in previous epoch",
     missing_all_validators=
@@ -224,7 +224,7 @@ proc process_justification_and_finalization*(
       checkpoint = shortLog(state.finalized_checkpoint),
       cat = "finalization"
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
   let
     total_balance = get_total_active_balance(state)
@@ -232,7 +232,7 @@ func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
   effective_balance * BASE_REWARD_FACTOR div
     integer_squareroot(total_balance) div BASE_REWARDS_PER_EPOCH
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
     tuple[a: seq[Gwei], b: seq[Gwei]] =
   let
@@ -326,7 +326,7 @@ func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
 
   (rewards, penalties)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func process_rewards_and_penalties(
     state: var BeaconState, cache: var StateCache) =
   if get_current_epoch(state) == GENESIS_EPOCH:
@@ -338,7 +338,7 @@ func process_rewards_and_penalties(
     increase_balance(state, i.ValidatorIndex, rewards[i])
     decrease_balance(state, i.ValidatorIndex, penalties[i])
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#slashings
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#slashings
 func process_slashings*(state: var BeaconState) =
   let
     epoch = get_current_epoch(state)
@@ -355,7 +355,7 @@ func process_slashings*(state: var BeaconState) =
       let penalty = penalty_numerator div total_balance * increment
       decrease_balance(state, index.ValidatorIndex, penalty)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#final-updates
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#final-updates
 proc process_final_updates*(state: var BeaconState) =
   let
     current_epoch = get_current_epoch(state)
@@ -402,7 +402,7 @@ proc process_final_updates*(state: var BeaconState) =
   state.previous_epoch_attestations = state.current_epoch_attestations
   state.current_epoch_attestations = @[]
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#epoch-processing
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#epoch-processing
 proc process_epoch*(state: var BeaconState) =
   # @proc are placeholders
 
@@ -411,16 +411,16 @@ proc process_epoch*(state: var BeaconState) =
 
   var per_epoch_cache = get_empty_per_epoch_cache()
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#justification-and-finalization
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#justification-and-finalization
   process_justification_and_finalization(state, per_epoch_cache)
 
   trace "ran process_justification_and_finalization",
     current_epoch = get_current_epoch(state)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
   process_rewards_and_penalties(state, per_epoch_cache)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#registry-updates
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#registry-updates
   # Don't rely on caching here.
   process_registry_updates(state)
 
@@ -433,12 +433,12 @@ proc process_epoch*(state: var BeaconState) =
   # @process_reveal_deadlines
   # @process_challenge_deadlines
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#slashings
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#slashings
   process_slashings(state)
 
   # @update_period_committee
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#final-updates
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#final-updates
   process_final_updates(state)
 
   # @after_process_final_updates
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index 9911acc9d..36cfe726d 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -11,8 +11,8 @@ import
   ./datatypes, ./digest, ./helpers
 
 # TODO: Proceed to renaming and signature changes
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_shuffled_index
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_shuffled_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_committee
 func get_shuffled_seq*(seed: Eth2Digest,
                        list_size: uint64,
                        ): seq[ValidatorIndex] =
@@ -78,7 +78,7 @@ func get_shuffled_seq*(seed: Eth2Digest,
 
   result = shuffled_active_validator_indices
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_previous_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_previous_epoch
 func get_previous_epoch*(state: BeaconState): Epoch =
   # Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
   let current_epoch = get_current_epoch(state)
@@ -121,7 +121,7 @@ func get_slot_and_index*(state: BeaconState, epoch: Epoch, shard: Shard): auto =
   # Want (slot % SLOTS_PER_EPOCH) * committees_per_slot + index to result in
   # same index, where
   # committees_per_slot = get_committee_count_at_slot(state, slot)
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_beacon_committee
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_beacon_committee
   let committees_per_slot =
     get_committee_count_at_slot(state, compute_start_slot_at_epoch(epoch))
 
@@ -135,7 +135,7 @@ func get_slot_and_index*(state: BeaconState, epoch: Epoch, shard: Shard): auto =
   # but not necessarily
   (compute_start_slot_at_epoch(epoch) + slot, index)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_committee
 func compute_committee(indices: seq[ValidatorIndex], seed: Eth2Digest,
     index: uint64, count: uint64, stateCache: var StateCache): seq[ValidatorIndex] =
   ## Return the committee corresponding to ``indices``, ``seed``, ``index``,
@@ -160,7 +160,7 @@ func compute_committee(indices: seq[ValidatorIndex], seed: Eth2Digest,
     start.int .. (endIdx.int-1),
     indices[stateCache.crosslink_committee_cache[key][it]])
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_beacon_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_beacon_committee
 func get_beacon_committee*(state: BeaconState, slot: Slot, index: uint64, cache: var StateCache): seq[ValidatorIndex] =
   # Return the beacon committee at ``slot`` for ``index``.
   let
@@ -209,7 +209,7 @@ func get_empty_per_epoch_cache*(): StateCache =
   result.start_shard_cache = initTable[Epoch, Shard]()
   result.committee_count_cache = initTable[Epoch, uint64]()
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_proposer_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#compute_proposer_index
 func compute_proposer_index*(state: BeaconState, indices: seq[ValidatorIndex],
     seed: Eth2Digest, stateCache: var StateCache): ValidatorIndex =
   # Return from ``indices`` a random index sampled by effective balance.
@@ -241,7 +241,7 @@ func compute_proposer_index*(state: BeaconState, indices: seq[ValidatorIndex],
       return candidate_index
     i += 1
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_beacon_proposer_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(state: BeaconState, stateCache: var StateCache):
     ValidatorIndex =
   # Return the beacon proposer index at the current slot.
diff --git a/beacon_chain/state_transition.nim b/beacon_chain/state_transition.nim
index 989c2c802..6c3ed6fb3 100644
--- a/beacon_chain/state_transition.nim
+++ b/beacon_chain/state_transition.nim
@@ -44,7 +44,7 @@ declareGauge beacon_previous_epoch_orphaned_blocks, "Number of blocks orphaned i
 # Canonical state transition functions
 # ---------------------------------------------------------------
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 func process_slot*(state: var BeaconState) =
   # Cache state root
   let previous_state_root = hash_tree_root(state)
@@ -81,7 +81,7 @@ func get_epoch_validator_count(state: BeaconState): int64 =
        validator.withdrawable_epoch > get_current_epoch(state):
       result += 1
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 proc process_slots*(state: var BeaconState, slot: Slot) =
   doAssert state.slot <= slot
 
@@ -97,7 +97,7 @@ proc process_slots*(state: var BeaconState, slot: Slot) =
     if is_epoch_transition:
       beacon_current_validators.set(get_epoch_validator_count(state))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 proc verifyStateRoot(state: BeaconState, blck: BeaconBlock): bool =
   # This is inlined in state_transition(...) in spec.
   let state_root = hash_tree_root(state)
@@ -171,7 +171,7 @@ proc state_transition*(
 # Hashed-state transition functions
 # ---------------------------------------------------------------
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 func process_slot(state: var HashedBeaconState) =
   # Cache state root
   let previous_slot_state_root = state.root
@@ -186,7 +186,7 @@ func process_slot(state: var HashedBeaconState) =
   state.data.block_roots[state.data.slot mod SLOTS_PER_HISTORICAL_ROOT] =
     signing_root(state.data.latest_block_header)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 proc process_slots*(state: var HashedBeaconState, slot: Slot) =
   # TODO: Eth specs strongly assert that state.data.slot <= slot
   #       This prevents receiving attestation in any order
diff --git a/beacon_chain/time.nim b/beacon_chain/time.nim
index a3003fc77..971b859e0 100644
--- a/beacon_chain/time.nim
+++ b/beacon_chain/time.nim
@@ -14,7 +14,7 @@ type
     ## which blocks are valid - in particular, blocks are not valid if they
     ## come from the future as seen from the local clock.
     ##
-    ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_fork-choice.md#fork-choice
+    ## https://github.com/ethereum/eth2.0-specs/blob/v0.9.1/specs/core/0_fork-choice.md#fork-choice
     ##
     # TODO replace time in chronos with a proper unit type, then this code can
     #      follow: