diff --git a/beacon_chain/attestation_pool.nim b/beacon_chain/attestation_pool.nim
index 5f3710cf2..4d5d0a855 100644
--- a/beacon_chain/attestation_pool.nim
+++ b/beacon_chain/attestation_pool.nim
@@ -161,13 +161,13 @@ proc slotIndex(
       attestationSlot =  $humaneSlotNum(attestationSlot)
     pool.startingSlot = state.finalized_epoch.get_epoch_start_slot()
 
-  if pool.startingSlot + pool.slots.len.Slot <= attestationSlot:
+  if pool.startingSlot + pool.slots.len.uint64 <= attestationSlot:
     debug "Growing attestation pool",
       attestationSlot =  $humaneSlotNum(attestationSlot),
       startingSlot = $humaneSlotNum(pool.startingSlot)
 
     # Make sure there's a pool entry for every slot, even when there's a gap
-    while pool.startingSlot + pool.slots.len.Slot <= attestationSlot:
+    while pool.startingSlot + pool.slots.len.uint64 <= attestationSlot:
       pool.slots.addLast(SlotData())
 
   if pool.startingSlot < state.finalized_epoch.get_epoch_start_slot():
@@ -193,7 +193,7 @@ proc add*(pool: var AttestationPool,
 
   let
     attestationSlot = attestation.data.slot
-    idx = pool.slotIndex(state, attestationSlot)
+    idx = pool.slotIndex(state, attestationSlot.Slot)
     slotData = addr pool.slots[idx]
     validation = Validation(
       aggregation_bitfield: attestation.aggregation_bitfield,
@@ -279,11 +279,11 @@ proc getAttestationsForBlock*(pool: AttestationPool,
     attestationSlot = newBlockSlot - MIN_ATTESTATION_INCLUSION_DELAY
 
   if attestationSlot < pool.startingSlot or
-      attestationSlot >= pool.startingSlot + pool.slots.len.Slot:
+      attestationSlot >= pool.startingSlot + pool.slots.len.uint64:
     info "No attestations",
       attestationSlot = humaneSlotNum(attestationSlot),
       startingSlot = humaneSlotNum(pool.startingSlot),
-      endingSlot = humaneSlotNum(pool.startingSlot + pool.slots.len.Slot)
+      endingSlot = humaneSlotNum(pool.startingSlot + pool.slots.len.uint64)
 
     return
 
diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index 67f37ff5d..b4e7af6e6 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -308,7 +308,7 @@ proc proposeBlock(node: BeaconNode,
   newBlock.state_root = Eth2Digest(data: hash_tree_root(state))
 
   let proposal = Proposal(
-    slot: slot,
+    slot: slot.uint64,
     shard: BEACON_CHAIN_SHARD_NUMBER,
     block_root: Eth2Digest(data: signed_root(newBlock, "signature")),
     signature: ValidatorSig(),
@@ -441,7 +441,7 @@ proc scheduleEpochActions(node: BeaconNode, epoch: Epoch) =
   let start = if epoch == GENESIS_EPOCH: 1.uint64 else: 0.uint64
 
   for i in start ..< SLOTS_PER_EPOCH:
-    let slot = epoch * SLOTS_PER_EPOCH + i
+    let slot = (epoch * SLOTS_PER_EPOCH + i).Slot
     nextState.slot = slot # ugly trick, see get_beacon_proposer_index
 
     block: # Schedule block proposals
diff --git a/beacon_chain/block_pool.nim b/beacon_chain/block_pool.nim
index fffc83dc3..c42a3c11d 100644
--- a/beacon_chain/block_pool.nim
+++ b/beacon_chain/block_pool.nim
@@ -54,7 +54,7 @@ proc init*(T: type BlockPool, db: BeaconChainDB): BlockPool =
   var blocksBySlot = initTable[uint64, seq[BlockRef]]()
   for _, b in tables.pairs(blocks):
     let slot = db.getBlock(b.root).get().slot
-    blocksBySlot.mgetOrPut(slot, @[]).add(b)
+    blocksBySlot.mgetOrPut(slot.uint64, @[]).add(b)
 
   BlockPool(
     pending: initTable[Eth2Digest, BeaconBlock](),
@@ -146,7 +146,7 @@ proc add*(
 
     pool.blocks[blockRoot] = blockRef
 
-    pool.addSlotMapping(blck.slot, blockRef)
+    pool.addSlotMapping(blck.slot.uint64, blockRef)
 
     # Resolved blocks should be stored in database
     pool.db.putBlock(blockRoot, blck)
@@ -229,8 +229,8 @@ proc getOrResolve*(pool: var BlockPool, root: Eth2Digest): BlockRef =
   if result.isNil:
     pool.unresolved[root] = UnresolvedBlock()
 
-iterator blockRootsForSlot*(pool: BlockPool, slot: uint64): Eth2Digest =
-  for br in pool.blocksBySlot.getOrDefault(slot, @[]):
+iterator blockRootsForSlot*(pool: BlockPool, slot: uint64|Slot): Eth2Digest =
+  for br in pool.blocksBySlot.getOrDefault(slot.uint64, @[]):
     yield br.root
 
 proc checkUnresolved*(pool: var BlockPool): seq[Eth2Digest] =
diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index accf57279..74bebaf68 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -280,7 +280,7 @@ func get_attestation_participants*(state: BeaconState,
   ## Return the participant indices at for the ``attestation_data`` and
   ## ``bitfield``.
   let crosslink_committees = get_crosslink_committees_at_slot(
-    state, attestation_data.slot)
+    state, attestation_data.slot.Slot)
   doAssert anyIt(
     crosslink_committees,
     it[1] == attestation_data.shard)
@@ -367,6 +367,8 @@ proc checkAttestation*(
   ## at the current slot. When acting as a proposer, the same rules need to
   ## be followed!
 
+  let attestation_data_slot = attestation.data.slot.Slot
+
   if not (attestation.data.slot >= GENESIS_SLOT):
     warn("Attestation predates genesis slot",
       attestation_slot = attestation.data.slot,
@@ -386,7 +388,7 @@ proc checkAttestation*(
     return
 
   let expected_justified_epoch =
-    if slot_to_epoch(attestation.data.slot + 1) >= get_current_epoch(state):
+    if slot_to_epoch(attestation.data.slot.Slot + 1) >= get_current_epoch(state):
       state.justified_epoch
     else:
       state.previous_justified_epoch
@@ -410,12 +412,12 @@ proc checkAttestation*(
       attestation.data.latest_crosslink,
       Crosslink(
         crosslink_data_root: attestation.data.crosslink_data_root,
-        epoch: slot_to_epoch(attestation.data.slot))]):
+        epoch: slot_to_epoch(attestation_data_slot))]):
     warn("Unexpected crosslink shard",
       state_latest_crosslinks_attestation_data_shard =
         state.latest_crosslinks[attestation.data.shard],
       attestation_data_latest_crosslink = attestation.data.latest_crosslink,
-      epoch = humaneEpochNum(slot_to_epoch(attestation.data.slot)),
+      epoch = humaneEpochNum(slot_to_epoch(attestation_data_slot)),
       crosslink_data_root = attestation.data.crosslink_data_root)
     return
 
@@ -423,7 +425,7 @@ proc checkAttestation*(
   assert anyIt(attestation.aggregation_bitfield, it != 0)
 
   let crosslink_committee = mapIt(
-    filterIt(get_crosslink_committees_at_slot(state, attestation.data.slot),
+    filterIt(get_crosslink_committees_at_slot(state, attestation_data_slot),
              it.shard == attestation.data.shard),
     it.committee)[0]
 
@@ -475,7 +477,7 @@ proc checkAttestation*(
           data: attestation.data, custody_bit: true)),
       ],
       attestation.aggregate_signature,
-      get_domain(state.fork, slot_to_epoch(attestation.data.slot),
+      get_domain(state.fork, slot_to_epoch(attestation.data.slot.Slot),
                  DOMAIN_ATTESTATION),
     )
 
@@ -513,7 +515,7 @@ proc makeAttestationData*(
     justified_block_root = get_block_root(state, justified_slot)
 
   AttestationData(
-    slot: state.slot,
+    slot: state.slot.uint64,
     shard: shard,
     beacon_block_root: beacon_block_root,
     epoch_boundary_root: epoch_boundary_root,
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index 68617c326..82999d176 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -18,7 +18,7 @@
 # types / composition
 
 import
-  eth/common, math,
+  eth/common, hashes, math,
   ./crypto, ./digest
 
 # TODO Data types:
@@ -39,6 +39,11 @@ import
 
 # TODO Many of these constants should go into a config object that can be used
 #      to run.. well.. a chain with different constants!
+
+type
+  Slot* = distinct uint64
+  Epoch* = uint64
+
 const
   SPEC_VERSION* = "0.4.0" ## \
   ## Spec version we're aiming to be compatible with, right now
@@ -103,8 +108,9 @@ const
   # Initial values
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#initial-values
   GENESIS_FORK_VERSION* = 0'u64
-  GENESIS_SLOT* = 2'u64^32
-  GENESIS_EPOCH* = GENESIS_SLOT div SLOTS_PER_EPOCH # slot_to_epoch(GENESIS_SLOT)
+  GENESIS_SLOT* = (2'u64^32).Slot
+  GENESIS_EPOCH* = (GENESIS_SLOT.uint64 div SLOTS_PER_EPOCH).Epoch ##\
+  ## slot_to_epoch(GENESIS_SLOT)
   GENESIS_START_SHARD* = 0'u64
   FAR_FUTURE_EPOCH* = not 0'u64 # 2^64 - 1 in spec
   ZERO_HASH* = Eth2Digest()
@@ -172,12 +178,6 @@ const
 type
   ValidatorIndex* = range[0'u32 .. 0xFFFFFF'u32] # TODO: wrap-around
 
-  ## In principle, these would be better as distinct types. That's a good
-  ## TODO eventually, but Danny has confirmed that the SSZ types will use
-  ## primitive (uint64, etc) types and helper functions annotated ones so
-  ## it would just create pointless casts for now.
-  Slot* = uint64
-  Epoch* = uint64
   Gwei* = uint64
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#proposerslashing
@@ -246,7 +246,7 @@ type
     latest_crosslink*: Crosslink ##\
     ## Last crosslink
 
-    justified_epoch*: uint64 ##\
+    justified_epoch*: Epoch ##\
     ## Last justified epoch in the beacon state
 
     justified_block_root*: Eth2Digest ##\
@@ -284,7 +284,7 @@ type
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#voluntaryexit
   VoluntaryExit* = object
     # Minimum epoch for processing exit
-    epoch*: uint64
+    epoch*: Epoch
     # Index of the exiting validator
     validator_index*: uint64
     # Validator signature
@@ -321,12 +321,12 @@ type
     ## Each block collects attestations, or votes, on past blocks, thus a chain
     ## is formed.
 
-    slot*: uint64
+    slot*: Slot
     parent_root*: Eth2Digest ##\
     ##\ Root hash of the previous block
 
     state_root*: Eth2Digest ##\
-    ##\ The state root, _after_ this block has been processed
+    ## The state root, _after_ this block has been processed
 
     randao_reveal*: ValidatorSig ##\
     ## Proposer RANDAO reveal
@@ -375,7 +375,7 @@ type
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#beaconstate
   BeaconState* = object
-    slot*: uint64
+    slot*: Slot
     genesis_time*: uint64
     fork*: Fork ##\
     ## For versioning hard forks
@@ -385,7 +385,7 @@ type
     validator_balances*: seq[uint64] ##\
     ## Validator balances in Gwei!
 
-    validator_registry_update_epoch*: uint64
+    validator_registry_update_epoch*: Epoch
 
     # TODO remove or conditionally compile; not in spec anymore
     validator_registry_delta_chain_tip*: Eth2Digest ##\
@@ -395,16 +395,16 @@ type
     latest_randao_mixes*: array[LATEST_BLOCK_ROOTS_LENGTH.int, Eth2Digest]
     previous_shuffling_start_shard*: uint64
     current_shuffling_start_shard*: uint64
-    previous_shuffling_epoch*: uint64
-    current_shuffling_epoch*: uint64
+    previous_shuffling_epoch*: Epoch
+    current_shuffling_epoch*: Epoch
     previous_shuffling_seed*: Eth2Digest
     current_shuffling_seed*: Eth2Digest
 
     # Finality
-    previous_justified_epoch*: uint64
-    justified_epoch*: uint64
+    previous_justified_epoch*: Epoch
+    justified_epoch*: Epoch
     justification_bitfield*: uint64
-    finalized_epoch*: uint64
+    finalized_epoch*: Epoch
 
     # Recent state
     latest_crosslinks*: array[SHARD_COUNT, Crosslink]
@@ -434,13 +434,13 @@ type
     withdrawal_credentials*: Eth2Digest ##\
     ## Withdrawal credentials
 
-    activation_epoch*: uint64 ##\
+    activation_epoch*: Epoch ##\
     ## Epoch when validator activated
 
-    exit_epoch*: uint64 ##\
+    exit_epoch*: Epoch ##\
     ## Epoch when validator exited
 
-    withdrawable_epoch*: uint64 ##\
+    withdrawable_epoch*: Epoch ##\
     ## Epoch when validator is eligible to withdraw
 
     initiated_exit*: bool ##\
@@ -451,7 +451,7 @@ type
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#crosslink
   Crosslink* = object
-    epoch*: uint64 ##\
+    epoch*: Epoch ##\
     ## Epoch number
 
     crosslink_data_root*: Eth2Digest ##\
@@ -462,13 +462,13 @@ type
     aggregation_bitfield*: seq[byte]          # Attester participation bitfield
     data*: AttestationData                    # Attestation data
     custody_bitfield*: seq[byte]              # Custody bitfield
-    inclusion_slot*: uint64                   # Inclusion slot
+    inclusion_slot*: Slot                     # Inclusion slot
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#fork
   Fork* = object
     previous_version*: uint64                     # Previous fork version
     current_version*: uint64                      # Current fork version
-    epoch*: uint64                                # Fork epoch number
+    epoch*: Epoch                                 # Fork epoch number
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#eth1data
   Eth1Data* = object
@@ -522,8 +522,39 @@ type
 func shortValidatorKey*(state: BeaconState, validatorIdx: int): string =
     ($state.validator_registry[validatorIdx].pubkey)[0..7]
 
-func humaneSlotNum*(s: Slot): Slot =
-  s - GENESIS_SLOT
+template ethTimeUnit(typ: type) =
+  proc `+`*(x: typ, y: uint64): typ {.borrow.}
+  proc `-`*(x: typ, y: uint64): typ {.borrow.}
+  proc `-`*(x: uint64, y: typ): typ {.borrow.}
+
+  proc `+=`*(x: var typ, y: uint64) {.borrow.}
+  proc `-=`*(x: var typ, y: uint64) {.borrow.}
+
+  # Not closed over type in question (Slot or Epoch)
+  proc `mod`*(x: typ, y: uint64): uint64 {.borrow.}
+  proc `div`*(x: typ, y: uint64): uint64 {.borrow.}
+  proc `div`*(x: uint64, y: typ): uint64 {.borrow.}
+  proc `-`*(x: typ, y: typ): uint64 {.borrow.}
+
+  # Comparison operators
+  proc `<`*(x: typ, y: typ): bool {.borrow.}
+  proc `<`*(x: typ, y: uint64): bool {.borrow.}
+  proc `<`*(x: uint64, y: typ): bool {.borrow.}
+  proc `<=`*(x: typ, y: typ): bool {.borrow.}
+  proc `<=`*(x: typ, y: uint64): bool {.borrow.}
+  proc `<=`*(x: uint64, y: typ): bool {.borrow.}
+  proc `==`*(x: typ, y: typ): bool {.borrow.}
+  proc `==`*(x: typ, y: uint64): bool {.borrow.}
+
+  # Nim integration
+  proc `$`*(x: typ): string {.borrow.}
+  proc hash*(x: typ): Hash {.borrow.}
+
+ethTimeUnit(Slot)
+#ethTimeUnit(Epoch)
+
+func humaneSlotNum*(s: auto): uint64 =
+  s.Slot - GENESIS_SLOT
 
 func humaneEpochNum*(e: Epoch): Epoch =
   e - GENESIS_EPOCH
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 6b4ee174f..3760fae48 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -120,13 +120,13 @@ func merkle_root*(values: openArray[Eth2Digest]): Eth2Digest =
   o[1]
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#slot_to_epoch
-func slot_to_epoch*(slot: Slot): Epoch =
-  slot div SLOTS_PER_EPOCH
+func slot_to_epoch*(slot: Slot|uint64): Epoch =
+  (slot div SLOTS_PER_EPOCH).Epoch
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#get_epoch_start_slot
 func get_epoch_start_slot*(epoch: Epoch): Slot =
   # Return the starting slot of the given ``epoch``.
-  epoch * SLOTS_PER_EPOCH
+  (epoch * SLOTS_PER_EPOCH).Slot
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#is_double_vote
 func is_double_vote*(attestation_data_1: AttestationData,
@@ -134,8 +134,9 @@ func is_double_vote*(attestation_data_1: AttestationData,
   ## Check if ``attestation_data_1`` and ``attestation_data_2`` have the same
   ## target.
   let
-    target_epoch_1 = slot_to_epoch(attestation_data_1.slot)
-    target_epoch_2 = slot_to_epoch(attestation_data_2.slot)
+    # RLP artifact
+    target_epoch_1 = slot_to_epoch(attestation_data_1.slot.Slot)
+    target_epoch_2 = slot_to_epoch(attestation_data_2.slot.Slot)
   target_epoch_1 == target_epoch_2
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#is_surround_vote
@@ -145,8 +146,9 @@ func is_surround_vote*(attestation_data_1: AttestationData,
   let
     source_epoch_1 = attestation_data_1.justified_epoch
     source_epoch_2 = attestation_data_2.justified_epoch
-    target_epoch_1 = slot_to_epoch(attestation_data_1.slot)
-    target_epoch_2 = slot_to_epoch(attestation_data_2.slot)
+    # RLP artifact
+    target_epoch_1 = slot_to_epoch(attestation_data_1.slot.Slot)
+    target_epoch_2 = slot_to_epoch(attestation_data_2.slot.Slot)
 
   source_epoch_1 < source_epoch_2 and target_epoch_2 < target_epoch_1
 
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index 0ade06e41..e235a6e50 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -126,7 +126,7 @@ func get_previous_epoch*(state: BeaconState): Epoch =
   max(get_current_epoch(state) - 1, GENESIS_EPOCH)
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#get_crosslink_committees_at_slot
-func get_crosslink_committees_at_slot*(state: BeaconState, slot: Slot,
+func get_crosslink_committees_at_slot*(state: BeaconState, slot: Slot|uint64,
                                        registry_change: bool = false):
     seq[CrosslinkCommittee] =
   ## Returns the list of ``(committee, shard)`` tuples for the ``slot``.
diff --git a/beacon_chain/ssz.nim b/beacon_chain/ssz.nim
index f1afeba4e..03c05974e 100644
--- a/beacon_chain/ssz.nim
+++ b/beacon_chain/ssz.nim
@@ -153,6 +153,12 @@ proc endRecord*(w: var SszWriter, memo: RecordWritingMemo) =
   let finalSize = uint32(w.stream.pos - memo.initialStreamPos - 4)
   memo.sizePrefixCursor.endWrite(finalSize.toBytesSSZ)
 
+func toSSZType(x: auto): auto =
+  when x is Slot:
+    x.uint64
+  else:
+    x
+
 proc writeValue*(w: var SszWriter, obj: auto) =
   # We are not using overloads here, because this leads to
   # slightly better error messages when the user provides
@@ -178,10 +184,10 @@ proc writeValue*(w: var SszWriter, obj: auto) =
         # for research/serialized_sizes, remove when appropriate
         when defined(debugFieldSizes) and obj is (BeaconState|BeaconBlock):
           let start = w.stream.pos
-          w.writeValue field
+          w.writeValue field.toSSZType
           debugEcho fieldName, ": ", w.stream.pos - start
         else:
-          w.writeValue field
+          w.writeValue field.toSSZType
     w.endRecord(memo)
 
 proc readValue*(r: var SszReader, result: var auto) =
@@ -235,7 +241,10 @@ proc readValue*(r: var SszReader, result: var auto) =
 
     else:
       result.deserializeFields(fieldName, field):
-        field = r.readValue(field.type)
+        when field is Slot:
+          field = r.readValue(uint64).Slot
+        else:
+          field = r.readValue(field.type)
 
     if r.stream[].pos != endPos:
       raise newException(CorruptedDataError, "SSZ includes unexpected bytes past the end of the deserialized object")
@@ -303,7 +312,7 @@ func hash_tree_root*[T: object|tuple](x: T): array[32, byte] =
   ## Containers have their fields recursively hashed, concatenated and hashed
   withHash:
     for field in x.fields:
-      h.update hash_tree_root(field)
+      h.update hash_tree_root(field.toSSZType)
 
 # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/simple-serialize.md#signed-roots
 func signed_root*[T: object](x: T, field_name: string): array[32, byte] =
@@ -316,7 +325,7 @@ func signed_root*[T: object](x: T, field_name: string): array[32, byte] =
       if name == field_name:
         found_field_name = true
         break
-      h.update hash_tree_root(field)
+      h.update hash_tree_root(field.toSSZType)
 
     doAssert found_field_name
 
diff --git a/beacon_chain/state_transition.nim b/beacon_chain/state_transition.nim
index 343797858..cb535cb60 100644
--- a/beacon_chain/state_transition.nim
+++ b/beacon_chain/state_transition.nim
@@ -49,7 +49,7 @@ func verifyBlockSignature(state: BeaconState, blck: BeaconBlock): bool =
     proposer =
       state.validator_registry[get_beacon_proposer_index(state, state.slot)]
     proposal = Proposal(
-      slot: blck.slot,
+      slot: blck.slot.uint64,
       shard: BEACON_CHAIN_SHARD_NUMBER,
       block_root: Eth2Digest(data: signed_root(blck, "signature")),
       signature: blck.signature)
@@ -361,7 +361,8 @@ proc processTransfers(state: var BeaconState, blck: BeaconBlock,
       let transfer_message = signed_root(transfer, "signature")
       if not bls_verify(
           pubkey=transfer.pubkey, transfer_message, transfer.signature,
-          get_domain(state.fork, slot_to_epoch(transfer.slot), DOMAIN_TRANSFER)):
+          get_domain(
+            state.fork, slot_to_epoch(transfer.slot), DOMAIN_TRANSFER)):
         notice "Transfer: incorrect signature"
         return false
 
@@ -611,7 +612,7 @@ func processEpoch(state: var BeaconState) =
   let
     previous_epoch_head_attestations =
       previous_epoch_attestations.filterIt(
-        it.data.beacon_block_root == get_block_root(state, it.data.slot))
+        it.data.beacon_block_root == get_block_root(state, it.data.slot.Slot))
 
     previous_epoch_head_attester_indices =
       toSet(get_attester_indices(state, previous_epoch_head_attestations))
@@ -708,8 +709,10 @@ func processEpoch(state: var BeaconState) =
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#crosslinks
   block:
-    for slot in get_epoch_start_slot(previous_epoch) ..< get_epoch_start_slot(next_epoch):
-      let crosslink_committees_at_slot = get_crosslink_committees_at_slot(state, slot)
+    for slot in get_epoch_start_slot(previous_epoch).uint64 ..<
+        get_epoch_start_slot(next_epoch).uint64:
+      let crosslink_committees_at_slot =
+        get_crosslink_committees_at_slot(state, slot.Slot)
 
       for crosslink_committee in crosslink_committees_at_slot:
         if 3'u64 * total_attesting_balance(crosslink_committee) >=
@@ -737,7 +740,7 @@ func processEpoch(state: var BeaconState) =
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#justification-and-finalization
   func inclusion_distances(state: BeaconState): auto =
-    result = initTable[ValidatorIndex, uint64]()
+    result = initTable[ValidatorIndex, Slot]()
 
     for a in previous_epoch_attestations:
       for v in get_attestation_participants(
@@ -801,7 +804,7 @@ func processEpoch(state: var BeaconState) =
         else:
           ## Last case will not occur. If this assumption becomes false,
           ## indexing into distances will fail.
-          initTable[ValidatorIndex, uint64]()
+          initTable[ValidatorIndex, Slot]()
 
       for index in active_validator_indices:
         if index notin previous_epoch_attester_indices:
@@ -837,7 +840,7 @@ func processEpoch(state: var BeaconState) =
     ## (reverse) index based on one O(n) scan of previous_epoch_attestations,
     ## then perform O(n) O(1) lookups. Keep same iteration order and rules in
     ## which first match wins and terminates for each ValidatorIndex.
-    var proposer_indexes = initTable[ValidatorIndex, uint64]()
+    var proposer_indexes = initTable[ValidatorIndex, Slot]()
 
     # This is the loop from inclusion_slot(...)
     for a in previous_epoch_attestations:
@@ -861,8 +864,10 @@ func processEpoch(state: var BeaconState) =
 
   # https://github.com/ethereum/eth2.0-specs/blob/0.4.0/specs/core/0_beacon-chain.md#crosslinks-1
   block:
-    for slot in get_epoch_start_slot(previous_epoch) ..< get_epoch_start_slot(current_epoch):
-      let crosslink_committees_at_slot = get_crosslink_committees_at_slot(state, slot)
+    for slot in get_epoch_start_slot(previous_epoch).uint64 ..<
+        get_epoch_start_slot(current_epoch).uint64:
+      let crosslink_committees_at_slot =
+        get_crosslink_committees_at_slot(state, slot.Slot)
       for crosslink_committee in crosslink_committees_at_slot:
         let
           committee_attesting_validators =
diff --git a/beacon_chain/sync_protocol.nim b/beacon_chain/sync_protocol.nim
index 026e41106..1a93d858c 100644
--- a/beacon_chain/sync_protocol.nim
+++ b/beacon_chain/sync_protocol.nim
@@ -24,7 +24,7 @@ type
 
 func toHeader(b: BeaconBlock): BeaconBlockHeader =
   BeaconBlockHeader(
-    slot: b.slot,
+    slot: b.slot.uint64,
     parent_root: b.parent_root,
     state_root: b.state_root,
     randao_reveal: b.randao_reveal,
@@ -35,7 +35,7 @@ func toHeader(b: BeaconBlock): BeaconBlockHeader =
 
 proc fromHeaderAndBody(b: var BeaconBlock, h: BeaconBlockHeader, body: BeaconBlockBody) =
   assert(hash_tree_root_final(body) == h.body)
-  b.slot = h.slot
+  b.slot = h.slot.Slot
   b.parent_root = h.parent_root
   b.state_root = h.state_root
   b.randao_reveal = h.randao_reveal
@@ -76,7 +76,7 @@ p2pProtocol BeaconSync(version = 1,
       latestFinalizedRoot: Eth2Digest # TODO
       latestFinalizedEpoch: uint64 = node.state.data.finalized_epoch
       bestRoot: Eth2Digest # TODO
-      bestSlot: uint64 = node.state.data.slot
+      bestSlot: uint64 = node.state.data.slot.uint64
 
     let m = await handshake(peer, timeout = 500,
                             status(networkId, latestFinalizedRoot,
diff --git a/research/serialized_sizes.nim b/research/serialized_sizes.nim
index 7aa85e2ba..468419cfc 100644
--- a/research/serialized_sizes.nim
+++ b/research/serialized_sizes.nim
@@ -15,7 +15,7 @@ proc stateSize(deposits: int, maxContent = false) =
     #      attestations it may hold, so we'll just add so many of them
     state.latest_attestations.setLen(MAX_ATTESTATIONS * SLOTS_PER_EPOCH * 2)
     let
-      crosslink_committees = get_crosslink_committees_at_slot(state, 0)
+      crosslink_committees = get_crosslink_committees_at_slot(state, 0.Slot)
       validatorsPerCommittee =
         len(crosslink_committees[0].committee) # close enough..
     for a in state.latest_attestations.mitems():
diff --git a/research/state_sim.nim b/research/state_sim.nim
index a63bc4756..e0264d94f 100644
--- a/research/state_sim.nim
+++ b/research/state_sim.nim
@@ -36,6 +36,8 @@ template withTimerRet(stats: var RunningStat, body: untyped): untyped =
 
   tmp
 
+proc `%`*(x: Slot): JsonNode {.borrow.}
+
 proc writeJson*(prefix, slot, v: auto) =
   var f: File
   defer: close(f)
@@ -80,7 +82,7 @@ cli do(slots = 1945,
     attestations[attestations_idx] = @[]
 
     let t =
-      if (state.slot + 2.Slot) mod SLOTS_PER_EPOCH == 0: tEpoch
+      if (state.slot + 2) mod SLOTS_PER_EPOCH == 0: tEpoch
       else: tBlock
 
     withTimer(timers[t]):
diff --git a/tests/test_beacon_chain_db.nim b/tests/test_beacon_chain_db.nim
index 30baf404c..609de1105 100644
--- a/tests/test_beacon_chain_db.nim
+++ b/tests/test_beacon_chain_db.nim
@@ -57,11 +57,12 @@ suite "Beacon chain DB":
     check: x == y
 
     let
-      a0 = BeaconBlock(slot: 0)
+      # TODO Not GENESIS_SLOT?
+      a0 = BeaconBlock(slot: 0.Slot)
       a0r = hash_tree_root_final(a0)
-      a1 = BeaconBlock(slot: 1, parent_root: a0r)
+      a1 = BeaconBlock(slot: 1.Slot, parent_root: a0r)
       a1r = hash_tree_root_final(a1)
-      a2 = BeaconBlock(slot: 2, parent_root: a1r)
+      a2 = BeaconBlock(slot: 2.Slot, parent_root: a1r)
       a2r = hash_tree_root_final(a2)
 
     doAssert toSeq(db.getAncestors(a0r)) == []
diff --git a/tests/test_ssz.nim b/tests/test_ssz.nim
index 3d093f64f..95fe90abb 100644
--- a/tests/test_ssz.nim
+++ b/tests/test_ssz.nim
@@ -74,8 +74,9 @@ suite "Simple serialization":
   SSZ.roundripTest [1, 2, 3]
   SSZ.roundripTest @[1, 2, 3]
   SSZ.roundripTest SigKey.random().getKey()
-  SSZ.roundripTest BeaconBlock(slot: 42, signature: sign(SigKey.random(), 0'u64, ""))
-  SSZ.roundripTest BeaconState(slot: 42)
+  SSZ.roundripTest BeaconBlock(
+    slot: 42.Slot, signature: sign(SigKey.random(), 0'u64, ""))
+  SSZ.roundripTest BeaconState(slot: 42.Slot)
 
 suite "Tree hashing":
   # TODO Nothing but smoke tests for now..
diff --git a/tests/testutil.nim b/tests/testutil.nim
index 6f4b0da05..719bd54c5 100644
--- a/tests/testutil.nim
+++ b/tests/testutil.nim
@@ -120,7 +120,7 @@ proc addBlock*(
     # Once we've collected all the state data, we sign the block data along with
     # some book-keeping values
     signed_data = Proposal(
-      slot: new_block.slot,
+      slot: new_block.slot.uint64,
       shard: BEACON_CHAIN_SHARD_NUMBER,
       block_root: Eth2Digest(data: signed_root(new_block, "signature")),
       signature: ValidatorSig(),