diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 9c26203a5f..ea8edc15b5 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -59,6 +59,7 @@
         - [`get_current_epoch`](#get_current_epoch)
         - [`get_epoch_start_slot`](#get_epoch_start_slot)
         - [`is_active_validator`](#is_active_validator)
+        - [`is_slashable_validator`](#is_slashable_validator)
         - [`get_active_validator_indices`](#get_active_validator_indices)
         - [`get_balance`](#get_balance)
         - [`set_balance`](#set_balance)
@@ -744,6 +745,18 @@ def is_active_validator(validator: Validator, epoch: Epoch) -> bool:
     return validator.activation_epoch <= epoch < validator.exit_epoch
 ```
 
+### `is_slashable_validator`
+```python
+def is_slashable_validator(validator: Validator, epoch: Epoch) -> bool:
+    """
+    Check if ``validator`` is slashable.
+    """
+    return (
+        validator.activation_epoch <= epoch < validator.withdrawable_epoch and
+        validator.slashed is False
+    )
+```
+
 ### `get_active_validator_indices`
 
 ```python
@@ -1427,7 +1440,6 @@ def slash_validator(state: BeaconState, index: ValidatorIndex) -> None:
     Note that this function mutates ``state``.
     """
     validator = state.validator_registry[index]
-    assert state.slot < get_epoch_start_slot(validator.withdrawable_epoch)  # [TO BE REMOVED IN PHASE 2]
     exit_validator(state, index)
     state.latest_slashed_balances[get_current_epoch(state) % LATEST_SLASHED_EXIT_LENGTH] += get_effective_balance(state, index)
 
@@ -2356,8 +2368,8 @@ def process_proposer_slashing(state: BeaconState,
     assert slot_to_epoch(proposer_slashing.header_1.slot) == slot_to_epoch(proposer_slashing.header_2.slot)
     # But the headers are different
     assert proposer_slashing.header_1 != proposer_slashing.header_2
-    # Proposer is not yet slashed
-    assert proposer.slashed is False
+    # Check proposer is slashable
+    assert is_slashable_validator(proposer, get_current_epoch(state))
     # Signatures are valid
     for header in (proposer_slashing.header_1, proposer_slashing.header_2):
         assert bls_verify(
@@ -2396,7 +2408,7 @@ def process_attester_slashing(state: BeaconState,
         index for index in attestation1.validator_indices
         if (
             index in attestation2.validator_indices and
-            state.validator_registry[index].slashed is False
+            is_slashable_validator(state.validator_registry[index], get_current_epoch(state))
         )
     ]
     assert len(slashable_indices) >= 1
diff --git a/tests/phase0/block_processing/test_process_proposer_slashing.py b/tests/phase0/block_processing/test_process_proposer_slashing.py
new file mode 100644
index 0000000000..467d2164ba
--- /dev/null
+++ b/tests/phase0/block_processing/test_process_proposer_slashing.py
@@ -0,0 +1,97 @@
+from copy import deepcopy
+import pytest
+
+import build.phase0.spec as spec
+from build.phase0.spec import (
+    get_balance,
+    get_current_epoch,
+    process_proposer_slashing,
+)
+from tests.phase0.helpers import (
+    get_valid_proposer_slashing,
+)
+
+# mark entire file as 'header'
+pytestmark = pytest.mark.proposer_slashings
+
+
+def run_proposer_slashing_processing(state, proposer_slashing, valid=True):
+    """
+    Run ``process_proposer_slashing`` returning the pre and post state.
+    If ``valid == False``, run expecting ``AssertionError``
+    """
+    post_state = deepcopy(state)
+
+    if not valid:
+        with pytest.raises(AssertionError):
+            process_proposer_slashing(post_state, proposer_slashing)
+        return state, None
+
+    process_proposer_slashing(post_state, proposer_slashing)
+
+    slashed_validator = post_state.validator_registry[proposer_slashing.proposer_index]
+    assert not slashed_validator.initiated_exit
+    assert slashed_validator.slashed
+    assert slashed_validator.exit_epoch < spec.FAR_FUTURE_EPOCH
+    assert slashed_validator.withdrawable_epoch < spec.FAR_FUTURE_EPOCH
+    # lost whistleblower reward
+    assert (
+        get_balance(post_state, proposer_slashing.proposer_index) <
+        get_balance(state, proposer_slashing.proposer_index)
+    )
+
+    return state, post_state
+
+
+def test_success(state):
+    proposer_slashing = get_valid_proposer_slashing(state)
+
+    pre_state, post_state = run_proposer_slashing_processing(state, proposer_slashing)
+
+    return pre_state, proposer_slashing, post_state
+
+
+def test_epochs_are_different(state):
+    proposer_slashing = get_valid_proposer_slashing(state)
+
+    # set slots to be in different epochs
+    proposer_slashing.header_2.slot += spec.SLOTS_PER_EPOCH
+
+    pre_state, post_state = run_proposer_slashing_processing(state, proposer_slashing, False)
+
+    return pre_state, proposer_slashing, post_state
+
+
+def test_headers_are_same(state):
+    proposer_slashing = get_valid_proposer_slashing(state)
+
+    # set headers to be the same
+    proposer_slashing.header_2 = proposer_slashing.header_1
+
+    pre_state, post_state = run_proposer_slashing_processing(state, proposer_slashing, False)
+
+    return pre_state, proposer_slashing, post_state
+
+
+def test_proposer_is_slashed(state):
+    proposer_slashing = get_valid_proposer_slashing(state)
+
+    # set proposer to slashed
+    state.validator_registry[proposer_slashing.proposer_index].slashed = True
+
+    pre_state, post_state = run_proposer_slashing_processing(state, proposer_slashing, False)
+
+    return pre_state, proposer_slashing, post_state
+
+
+def test_proposer_is_withdrawn(state):
+    proposer_slashing = get_valid_proposer_slashing(state)
+
+    # set proposer withdrawable_epoch in past
+    current_epoch = get_current_epoch(state)
+    proposer_index = proposer_slashing.proposer_index
+    state.validator_registry[proposer_index].withdrawable_epoch = current_epoch - 1
+
+    pre_state, post_state = run_proposer_slashing_processing(state, proposer_slashing, False)
+
+    return pre_state, proposer_slashing, post_state
diff --git a/tests/phase0/helpers.py b/tests/phase0/helpers.py
index 5c61685a62..3987289bff 100644
--- a/tests/phase0/helpers.py
+++ b/tests/phase0/helpers.py
@@ -7,14 +7,18 @@
 from build.phase0.spec import (
     # constants
     EMPTY_SIGNATURE,
+    ZERO_HASH,
     # SSZ
     AttestationData,
+    BeaconBlockHeader,
     Deposit,
     DepositInput,
     DepositData,
     Eth1Data,
+    ProposerSlashing,
     VoluntaryExit,
     # functions
+    get_active_validator_indices,
     get_block_root,
     get_current_epoch,
     get_domain,
@@ -199,3 +203,43 @@ def build_deposit(state,
     )
 
     return deposit, root, deposit_data_leaves
+
+
+def get_valid_proposer_slashing(state):
+    current_epoch = get_current_epoch(state)
+    validator_index = get_active_validator_indices(state.validator_registry, current_epoch)[-1]
+    privkey = pubkey_to_privkey[state.validator_registry[validator_index].pubkey]
+    slot = state.slot
+
+    header_1 = BeaconBlockHeader(
+        slot=slot,
+        previous_block_root=ZERO_HASH,
+        state_root=ZERO_HASH,
+        block_body_root=ZERO_HASH,
+        signature=EMPTY_SIGNATURE,
+    )
+    header_2 = deepcopy(header_1)
+    header_2.previous_block_root = b'\x02' * 32
+    header_2.slot = slot + 1
+
+    domain = get_domain(
+        fork=state.fork,
+        epoch=get_current_epoch(state),
+        domain_type=spec.DOMAIN_BEACON_BLOCK,
+    )
+    header_1.signature = bls.sign(
+        message_hash=signed_root(header_1),
+        privkey=privkey,
+        domain=domain,
+    )
+    header_2.signature = bls.sign(
+        message_hash=signed_root(header_2),
+        privkey=privkey,
+        domain=domain,
+    )
+
+    return ProposerSlashing(
+        proposer_index=validator_index,
+        header_1=header_1,
+        header_2=header_2,
+    )
diff --git a/tests/phase0/test_sanity.py b/tests/phase0/test_sanity.py
index ec03fb3555..444075a133 100644
--- a/tests/phase0/test_sanity.py
+++ b/tests/phase0/test_sanity.py
@@ -46,6 +46,7 @@
     build_deposit_data,
     build_empty_block_for_next_slot,
     force_registry_change_at_next_epoch,
+    get_valid_proposer_slashing,
 )
 
 
@@ -117,42 +118,8 @@ def test_empty_epoch_transition_not_finalizing(state):
 
 def test_proposer_slashing(state, pubkeys, privkeys):
     test_state = deepcopy(state)
-    current_epoch = get_current_epoch(test_state)
-    validator_index = get_active_validator_indices(test_state.validator_registry, current_epoch)[-1]
-    privkey = privkeys[validator_index]
-    slot = spec.GENESIS_SLOT
-    header_1 = BeaconBlockHeader(
-        slot=slot,
-        previous_block_root=ZERO_HASH,
-        state_root=ZERO_HASH,
-        block_body_root=ZERO_HASH,
-        signature=EMPTY_SIGNATURE,
-    )
-    header_2 = deepcopy(header_1)
-    header_2.previous_block_root = b'\x02' * 32
-    header_2.slot = slot + 1
-
-    domain = get_domain(
-        fork=test_state.fork,
-        epoch=get_current_epoch(test_state),
-        domain_type=spec.DOMAIN_BEACON_BLOCK,
-    )
-    header_1.signature = bls.sign(
-        message_hash=signed_root(header_1),
-        privkey=privkey,
-        domain=domain,
-    )
-    header_2.signature = bls.sign(
-        message_hash=signed_root(header_2),
-        privkey=privkey,
-        domain=domain,
-    )
-
-    proposer_slashing = ProposerSlashing(
-        proposer_index=validator_index,
-        header_1=header_1,
-        header_2=header_2,
-    )
+    proposer_slashing = get_valid_proposer_slashing(state)
+    validator_index = proposer_slashing.proposer_index
 
     #
     # Add to state via block transition