diff --git a/specs/phase1/beacon-chain.md b/specs/phase1/beacon-chain.md
index 049edce8e..81a8ddcc8 100644
--- a/specs/phase1/beacon-chain.md
+++ b/specs/phase1/beacon-chain.md
@@ -46,6 +46,7 @@
     - [`compute_updated_gasprice`](#compute_updated_gasprice)
     - [`compute_committee_source_epoch`](#compute_committee_source_epoch)
   - [Beacon state accessors](#beacon-state-accessors)
+    - [Updated `get_committee_count_per_slot`](#updated-get_committee_count_per_slot)
     - [`get_active_shard_count`](#get_active_shard_count)
     - [`get_online_validator_indices`](#get_online_validator_indices)
     - [`get_shard_committee`](#get_shard_committee)
@@ -105,6 +106,7 @@ Configuration is not namespaced. Instead it is strictly an extension;
 | Name | Value |
 | - | - | 
 | `MAX_SHARDS` | `2**10` (= 1024) |
+| `INITIAL_ACTIVE_SHARDS` | `2**6` (= 64) |
 | `LIGHT_CLIENT_COMMITTEE_SIZE` | `2**7` (= 128) |
 | `GASPRICE_ADJUSTMENT_COEFFICIENT` | `2**3` (= 8) | 
 
@@ -510,11 +512,28 @@ def compute_committee_source_epoch(epoch: Epoch, period: uint64) -> Epoch:
 
 ### Beacon state accessors
 
+#### Updated `get_committee_count_per_slot`
+
+```python
+def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
+    """
+    Return the number of committees in each slot for the given ``epoch``.
+    """
+    return max(uint64(1), min(
+        get_active_shard_count(state),
+        uint64(len(get_active_validator_indices(state, epoch))) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE,
+    ))
+```
+
 #### `get_active_shard_count`
 
 ```python
 def get_active_shard_count(state: BeaconState) -> uint64:
-    return len(state.shard_states)  # May adapt in the future, or change over time.
+    """
+    Return the number of active shards.
+    Note that this puts an upper bound on the number of committees per slot.
+    """
+    return INITIAL_ACTIVE_SHARDS
 ```
 
 #### `get_online_validator_indices`
@@ -535,12 +554,11 @@ def get_shard_committee(beacon_state: BeaconState, epoch: Epoch, shard: Shard) -
     source_epoch = compute_committee_source_epoch(epoch, SHARD_COMMITTEE_PERIOD)
     active_validator_indices = get_active_validator_indices(beacon_state, source_epoch)
     seed = get_seed(beacon_state, source_epoch, DOMAIN_SHARD_COMMITTEE)
-    active_shard_count = get_active_shard_count(beacon_state)
     return compute_committee(
         indices=active_validator_indices,
         seed=seed,
         index=shard,
-        count=active_shard_count,
+        count=get_active_shard_count(beacon_state),
     )
 ```
 
@@ -607,10 +625,11 @@ def get_start_shard(state: BeaconState, slot: Slot) -> Shard:
     else:
         # Previous epoch
         shard_delta = get_committee_count_delta(state, start_slot=slot, stop_slot=current_epoch_start_slot)
-        max_committees_per_epoch = MAX_COMMITTEES_PER_SLOT * SLOTS_PER_EPOCH
+        max_committees_per_slot = active_shard_count
+        max_committees_in_span = max_committees_per_slot * (current_epoch_start_slot - slot)
         return Shard(
             # Ensure positive
-            (state.current_epoch_start_shard + max_committees_per_epoch * active_shard_count - shard_delta)
+            (state.current_epoch_start_shard + max_committees_in_span - shard_delta)
             % active_shard_count
         )
 ```
@@ -742,7 +761,6 @@ def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
 def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
     data = attestation.data
     assert data.index < get_committee_count_per_slot(state, data.target.epoch)
-    assert data.index < get_active_shard_count(state)
     assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
     assert data.target.epoch == compute_epoch_at_slot(data.slot)
     assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH
diff --git a/specs/phase1/phase1-fork.md b/specs/phase1/phase1-fork.md
index e3949aca8..a094b01fc 100644
--- a/specs/phase1/phase1-fork.md
+++ b/specs/phase1/phase1-fork.md
@@ -36,7 +36,6 @@ Warning: this configuration is not definitive.
 | - | - |
 | `PHASE_1_FORK_VERSION` | `Version('0x01000000')` |
 | `PHASE_1_FORK_SLOT` | `Slot(0)` **TBD** |
-| `INITIAL_ACTIVE_SHARDS` | `2**6` (= 64) |
 
 ## Fork to Phase 1
 
diff --git a/tests/core/pyspec/eth2spec/test/conftest.py b/tests/core/pyspec/eth2spec/test/conftest.py
index 21f7c7abb..b961ee082 100644
--- a/tests/core/pyspec/eth2spec/test/conftest.py
+++ b/tests/core/pyspec/eth2spec/test/conftest.py
@@ -31,7 +31,7 @@ def pytest_addoption(parser):
         help="config: make the pyspec use the specified configuration"
     )
     parser.addoption(
-        "--disable-bls", action="store_true",
+        "--disable-bls", action="store_true", default=False,
         help="bls-default: make tests that are not dependent on BLS run without BLS"
     )
     parser.addoption(
diff --git a/tests/core/pyspec/eth2spec/test/context.py b/tests/core/pyspec/eth2spec/test/context.py
index 68696ea0a..69be2fd0a 100644
--- a/tests/core/pyspec/eth2spec/test/context.py
+++ b/tests/core/pyspec/eth2spec/test/context.py
@@ -151,6 +151,15 @@ def low_single_balance(spec):
     return [1]
 
 
+def large_validator_set(spec):
+    """
+    Helper method to create a series of default balances.
+    Usage: `@with_custom_state(balances_fn=default_balances, ...)`
+    """
+    num_validators = 2 * spec.SLOTS_PER_EPOCH * spec.MAX_COMMITTEES_PER_SLOT * spec.TARGET_COMMITTEE_SIZE
+    return [spec.MAX_EFFECTIVE_BALANCE] * num_validators
+
+
 def single_phase(fn):
     """
     Decorator that filters out the phases data.
diff --git a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
index 6d7ff096a..5b98abac4 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
@@ -22,7 +22,10 @@ from eth2spec.test.helpers.shard_transitions import get_shard_transition_of_comm
 
 from eth2spec.test.context import (
     PHASE0, PHASE1,
+    spec_test,
     spec_state_test, with_all_phases, expect_assertion_error, always_bls, with_phases,
+    with_custom_state, single_phase,
+    large_validator_set,
 )
 
 
@@ -70,6 +73,7 @@ def test_same_slot_block_transition(spec, state):
 def test_empty_block_transition(spec, state):
     pre_slot = state.slot
     pre_eth1_votes = len(state.eth1_data_votes)
+    pre_mix = spec.get_randao_mix(state, spec.get_current_epoch(state))
 
     yield 'pre', state
 
@@ -82,7 +86,30 @@ def test_empty_block_transition(spec, state):
 
     assert len(state.eth1_data_votes) == pre_eth1_votes + 1
     assert spec.get_block_root_at_slot(state, pre_slot) == signed_block.message.parent_root
-    assert spec.get_randao_mix(state, spec.get_current_epoch(state)) != spec.Bytes32()
+    assert spec.get_randao_mix(state, spec.get_current_epoch(state)) != pre_mix
+
+
+@with_all_phases
+@spec_test
+@with_custom_state(balances_fn=large_validator_set, threshold_fn=lambda spec: spec.EJECTION_BALANCE)
+@single_phase
+def test_empty_block_transition_large_validator_set(spec, state):
+    pre_slot = state.slot
+    pre_eth1_votes = len(state.eth1_data_votes)
+    pre_mix = spec.get_randao_mix(state, spec.get_current_epoch(state))
+
+    yield 'pre', state
+
+    block = build_empty_block_for_next_slot(spec, state)
+
+    signed_block = state_transition_and_sign_block(spec, state, block)
+
+    yield 'blocks', [signed_block]
+    yield 'post', state
+
+    assert len(state.eth1_data_votes) == pre_eth1_votes + 1
+    assert spec.get_block_root_at_slot(state, pre_slot) == signed_block.message.parent_root
+    assert spec.get_randao_mix(state, spec.get_current_epoch(state)) != pre_mix
 
 
 def process_and_sign_block_without_header_validations(spec, state, block):
@@ -288,6 +315,26 @@ def test_empty_epoch_transition(spec, state):
         assert spec.get_block_root_at_slot(state, slot) == block.parent_root
 
 
+@with_all_phases
+@spec_test
+@with_custom_state(balances_fn=large_validator_set, threshold_fn=lambda spec: spec.EJECTION_BALANCE)
+@single_phase
+def test_empty_epoch_transition_large_validator_set(spec, state):
+    pre_slot = state.slot
+    yield 'pre', state
+
+    block = build_empty_block(spec, state, state.slot + spec.SLOTS_PER_EPOCH)
+
+    signed_block = state_transition_and_sign_block(spec, state, block)
+
+    yield 'blocks', [signed_block]
+    yield 'post', state
+
+    assert state.slot == block.slot
+    for slot in range(pre_slot, state.slot):
+        assert spec.get_block_root_at_slot(state, slot) == block.parent_root
+
+
 @with_all_phases
 @spec_state_test
 def test_empty_epoch_transition_not_finalizing(spec, state):
diff --git a/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py b/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
index c443545b9..a802d6c3c 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/unittests/test_get_start_shard.py
@@ -74,3 +74,16 @@ def test_get_start_shard_previous_slot(spec, state):
         - spec.get_committee_count_delta(state, start_slot=slot, stop_slot=current_epoch_start_slot)
     ) % active_shard_count
     assert start_shard == expected_start_shard
+
+
+@with_all_phases_except([PHASE0])
+@spec_state_test
+def test_get_start_shard_far_past_epoch(spec, state):
+    initial_epoch = spec.get_current_epoch(state)
+    initial_start_slot = spec.compute_start_slot_at_epoch(initial_epoch)
+    initial_start_shard = state.current_epoch_start_shard
+
+    for _ in range(spec.MAX_SHARDS + 2):
+        next_epoch(spec, state)
+
+    assert spec.get_start_shard(state, initial_start_slot) == initial_start_shard