diff --git a/tests/core/pyspec/eth2spec/test/helpers/block.py b/tests/core/pyspec/eth2spec/test/helpers/block.py
index b8f7c4bcb..7daa51970 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/block.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/block.py
@@ -4,6 +4,7 @@ from eth2spec.test.helpers.keys import privkeys
 from eth2spec.utils import bls
 from eth2spec.utils.bls import only_with_bls
 from eth2spec.utils.ssz.ssz_impl import hash_tree_root
+from eth2spec.utils.ssz.ssz_typing import uint256
 
 
 def get_proposer_index_maybe(spec, state, slot, proposer_index=None):
@@ -122,3 +123,14 @@ def get_state_and_beacon_parent_root_at_slot(spec, state, slot):
         previous_block_header.state_root = hash_tree_root(state)
     beacon_parent_root = hash_tree_root(previous_block_header)
     return state, beacon_parent_root
+
+
+def prepare_empty_pow_block(spec):
+    return spec.PowBlock(
+        block_hash=spec.Hash32(),
+        parent_hash=spec.Hash32(),
+        is_processed=False,
+        is_valid=True,
+        total_difficulty=uint256(0),
+        difficulty=uint256(0)
+    )
diff --git a/tests/core/pyspec/eth2spec/test/helpers/execution_payload.py b/tests/core/pyspec/eth2spec/test/helpers/execution_payload.py
index 43be965a5..d82a6a54d 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/execution_payload.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/execution_payload.py
@@ -1,3 +1,6 @@
+from remerkleable.byte_arrays import ByteVector
+
+
 def build_empty_execution_payload(spec, state, randao_mix=None):
     """
     Assuming a pre-state of the same slot, build a valid ExecutionPayload without any transactions.
@@ -64,3 +67,12 @@ def build_state_with_execution_payload_header(spec, state, execution_payload_hea
     pre_state.latest_execution_payload_header = execution_payload_header
 
     return pre_state
+
+
+# damages last byte of the data by changing one bit
+def screw_up_bytes(data: ByteVector):
+    assert len(data) > 0
+    length = data.vector_length()
+    raw_data = data.encode_bytes()
+    raw_data = raw_data[0:len(raw_data) - 1] + bytes([(raw_data[len(raw_data) - 1] ^ 1)])
+    return ByteVector[length](*raw_data)
diff --git a/tests/core/pyspec/eth2spec/test/merge/block_processing/test_process_execution_payload.py b/tests/core/pyspec/eth2spec/test/merge/block_processing/test_process_execution_payload.py
index 4c68034d4..a2af7dc92 100644
--- a/tests/core/pyspec/eth2spec/test/merge/block_processing/test_process_execution_payload.py
+++ b/tests/core/pyspec/eth2spec/test/merge/block_processing/test_process_execution_payload.py
@@ -1,8 +1,12 @@
+from remerkleable.byte_arrays import Bytes32
+from eth2spec.utils.ssz.ssz_typing import uint64
+
 from eth2spec.test.helpers.execution_payload import (
     build_empty_execution_payload,
     get_execution_payload_header,
     build_state_with_incomplete_transition,
     build_state_with_complete_transition,
+    screw_up_bytes
 )
 from eth2spec.test.context import spec_state_test, expect_assertion_error, with_merge_and_later
 from eth2spec.test.helpers.state import next_slot
@@ -199,3 +203,182 @@ def test_bad_timestamp_regular_payload(spec, state):
     execution_payload.timestamp = execution_payload.timestamp + 1
 
     yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_bad_randao_first_payload(spec, state):
+    # pre-state
+    state = build_state_with_incomplete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    good_randao: Bytes32 = execution_payload.random
+    bad_randao = screw_up_bytes(good_randao)
+    # still valid because randao is ignored on this stage
+    execution_payload.random = bad_randao
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_bad_randao_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    good_randao: Bytes32 = execution_payload.random
+    bad_randao = screw_up_bytes(good_randao)
+    execution_payload.random = bad_randao
+
+    yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_zero_first_payload(spec, state):
+    # pre-state
+    state = build_state_with_incomplete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = uint64(0)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_max_first_payload(spec, state):
+    # pre-state
+    state = build_state_with_incomplete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = uint64(2**64 - 1)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_upper_plus_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit + \
+                                  execution_payload.gas_limit // spec.GAS_LIMIT_DENOMINATOR
+
+    yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_upper_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit + \
+                                  execution_payload.gas_limit // spec.GAS_LIMIT_DENOMINATOR - uint64(1)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_lower_minus_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit - \
+                                  execution_payload.gas_limit // spec.GAS_LIMIT_DENOMINATOR
+
+    yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_lower_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit - \
+                                  execution_payload.gas_limit // spec.GAS_LIMIT_DENOMINATOR + uint64(1)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_minimum_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+    state.latest_execution_payload_header.gas_limit = spec.MIN_GAS_LIMIT
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gaslimit_minimum_minus_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+    state.latest_execution_payload_header.gas_limit = spec.MIN_GAS_LIMIT
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_limit = execution_payload.gas_limit - uint64(1)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gasused_gaslimit_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_used = execution_payload.gas_limit
+
+    yield from run_execution_payload_processing(spec, state, execution_payload)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_gasused_gaslimit_plus_regular_payload(spec, state):
+    # pre-state
+    state = build_state_with_complete_transition(spec, state)
+    next_slot(spec, state)
+
+    # execution payload
+    execution_payload = build_empty_execution_payload(spec, state)
+    execution_payload.gas_used = execution_payload.gas_limit + uint64(1)
+
+    yield from run_execution_payload_processing(spec, state, execution_payload, valid=False)
diff --git a/tests/core/pyspec/eth2spec/test/merge/fork_choice/__init__.py b/tests/core/pyspec/eth2spec/test/merge/fork_choice/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/core/pyspec/eth2spec/test/merge/fork_choice/test_on_block.py b/tests/core/pyspec/eth2spec/test/merge/fork_choice/test_on_block.py
new file mode 100644
index 000000000..f096fb138
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/merge/fork_choice/test_on_block.py
@@ -0,0 +1,174 @@
+from eth2spec.utils.ssz.ssz_typing import uint64, uint256
+from eth2spec.test.helpers.block import (
+    prepare_empty_pow_block
+)
+from eth2spec.test.context import spec_state_test, expect_assertion_error, with_merge_and_later
+
+
+def create_transition_store(spec):
+    anchor_block = prepare_empty_pow_block(spec)
+    transition_store = spec.get_transition_store(anchor_block)
+    return transition_store
+
+
+class BlockNotFoundException(Exception):
+    pass
+
+
+def run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload,
+                                        valid=True, block_lookup_success=True):
+    """
+    Run ``process_merge_execution_payload``, yielding:
+      - transition store ('transition_store')
+      - current block ('block')
+      - parent block ('parent_block')
+      - execution payload ('payload')
+    If ``valid == False``, run expecting ``AssertionError``
+    If ``block_lookup_success == False``, run expecting ``BlockNotFoundException``
+    """
+
+    yield 'transition_store', transition_store
+    yield 'block', block
+    yield 'parent_block', parent_block
+    yield 'payload', payload
+
+    def get_pow_block(hash: spec.Bytes32) -> spec.PowBlock:
+        if hash == block.block_hash:
+            return block
+        elif hash == parent_block.block_hash:
+            return parent_block
+        else:
+            raise BlockNotFoundException()
+    save_pow_block = spec.get_pow_block
+
+    # Guido authorized everyone to do this
+    spec.get_pow_block = get_pow_block
+    exception_caught = False
+    block_not_found_exception_caught = False
+    try:
+        spec.process_merge_execution_payload(transition_store, payload)
+    except BlockNotFoundException:
+        block_not_found_exception_caught = True
+    except AssertionError:
+        exception_caught = True
+    except Exception as e:
+        spec.get_pow_block = save_pow_block
+        raise e
+    spec.get_pow_block = save_pow_block
+
+    if block_lookup_success:
+        assert not block_not_found_exception_caught
+    else:
+        assert block_not_found_exception_caught
+    if valid:
+        assert not exception_caught
+    else:
+        assert exception_caught
+
+
+@with_merge_and_later
+@spec_state_test
+def test_valid_terminal_pow_block_success_valid_fail_invalid(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty - uint256(1)
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.total_difficulty = transition_store.terminal_total_difficulty
+
+    assert spec.is_valid_terminal_pow_block(transition_store, block, parent_block)
+
+    block.is_valid = False
+    assert not spec.is_valid_terminal_pow_block(transition_store, block, parent_block)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_valid_terminal_pow_block_fail_before_terminal(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty - uint256(2)
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.total_difficulty = transition_store.terminal_total_difficulty - uint256(1)
+
+    assert not spec.is_valid_terminal_pow_block(transition_store, block, parent_block)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_valid_terminal_pow_block_fail_just_after_terminal(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.total_difficulty = transition_store.terminal_total_difficulty + uint256(1)
+
+    assert not spec.is_valid_terminal_pow_block(transition_store, block, parent_block)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_process_merge_execution_payload_success(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.block_hash = spec.Hash32(spec.hash(b'01'))
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty - uint256(1)
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.is_processed = True
+    block.total_difficulty = transition_store.terminal_total_difficulty
+    payload = spec.ExecutionPayload()
+    payload.parent_hash = block.block_hash
+    yield from run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload)
+    block.is_processed = False
+    yield from run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload, valid=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_process_merge_execution_payload_fail_block_lookup(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.block_hash = spec.Hash32(spec.hash(b'01'))
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty - uint256(1)
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.is_processed = True
+    block.total_difficulty = transition_store.terminal_total_difficulty
+    payload = spec.ExecutionPayload()
+    payload.parent_hash = spec.Hash32(spec.hash(b'02'))
+    yield from run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload, block_lookup_success=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_process_merge_execution_payload_fail_parent_block_lookup(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.block_hash = spec.Hash32(spec.hash(b'01'))
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty - uint256(1)
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = spec.Hash32(spec.hash(b'00'))
+    block.is_processed = True
+    block.total_difficulty = transition_store.terminal_total_difficulty
+    payload = spec.ExecutionPayload()
+    payload.parent_hash = block.block_hash
+    yield from run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload, block_lookup_success=False)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_process_merge_execution_payload_fail_after_terminal(spec, state):
+    transition_store = create_transition_store(spec)
+    parent_block = prepare_empty_pow_block(spec)
+    parent_block.block_hash = spec.Hash32(spec.hash(b'01'))
+    parent_block.total_difficulty = transition_store.terminal_total_difficulty
+    block = prepare_empty_pow_block(spec)
+    block.parent_hash = parent_block.block_hash
+    block.is_processed = True
+    block.total_difficulty = transition_store.terminal_total_difficulty + 1
+    payload = spec.ExecutionPayload()
+    payload.parent_hash = block.block_hash
+    yield from run_process_merge_execution_payload(spec, transition_store, block, parent_block, payload, valid=False)
\ No newline at end of file
diff --git a/tests/core/pyspec/eth2spec/test/merge/unit/__init__.py b/tests/core/pyspec/eth2spec/test/merge/unit/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/core/pyspec/eth2spec/test/merge/unit/test_unit.py b/tests/core/pyspec/eth2spec/test/merge/unit/test_unit.py
new file mode 100644
index 000000000..24caa03b1
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/merge/unit/test_unit.py
@@ -0,0 +1,121 @@
+from eth2spec.utils.ssz.ssz_typing import uint64, uint256
+from eth2spec.test.helpers.execution_payload import (
+    build_empty_execution_payload,
+    build_state_with_incomplete_transition,
+    build_state_with_complete_transition,
+)
+from eth2spec.test.helpers.block import (
+    prepare_empty_pow_block
+)
+from eth2spec.test.context import spec_state_test, expect_assertion_error, with_merge_and_later
+
+
+@with_merge_and_later
+@spec_state_test
+def test_fail_merge_complete(spec, state):
+    state = build_state_with_incomplete_transition(spec, state)
+    assert not spec.is_merge_complete(state)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_success_merge_complete(spec, state):
+    state = build_state_with_complete_transition(spec, state)
+    assert spec.is_merge_complete(state)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_fail_merge_block_false_false(spec, state):
+    state = build_state_with_complete_transition(spec, state)
+    execution_payload = spec.ExecutionPayload()
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert not spec.is_merge_block(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_fail_merge_block_false_true(spec, state):
+    state = build_state_with_complete_transition(spec, state)
+    execution_payload = build_empty_execution_payload(spec, state)
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert not spec.is_merge_block(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_fail_merge_block_true_false(spec, state):
+    state = build_state_with_incomplete_transition(spec, state)
+    execution_payload = spec.ExecutionPayload()
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert not spec.is_merge_block(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_success_merge_block(spec, state):
+    state = build_state_with_incomplete_transition(spec, state)
+    execution_payload = build_empty_execution_payload(spec, state)
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert spec.is_merge_block(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_fail_execution_enabled_false_false(spec, state):
+    state = build_state_with_incomplete_transition(spec, state)
+    execution_payload = spec.ExecutionPayload()
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert not spec.is_execution_enabled(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_success_execution_enabled_true_false(spec, state):
+    state = build_state_with_incomplete_transition(spec, state)
+    execution_payload = build_empty_execution_payload(spec, state)
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert spec.is_execution_enabled(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_success_execution_enabled_false_true(spec, state):
+    state = build_state_with_complete_transition(spec, state)
+    execution_payload = spec.ExecutionPayload()
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert spec.is_execution_enabled(state, body)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_success_execution_enabled_true_true(spec, state):
+    state = build_state_with_complete_transition(spec, state)
+    execution_payload = build_empty_execution_payload(spec, state)
+    body = spec.BeaconBlockBody()
+    body.execution_payload = execution_payload
+    assert spec.is_execution_enabled(state, body)
+
+
+def compute_terminal_total_difficulty_reference(spec) -> uint256:
+    seconds_per_voting_period = spec.EPOCHS_PER_ETH1_VOTING_PERIOD * spec.SLOTS_PER_EPOCH * spec.config.SECONDS_PER_SLOT
+    pow_blocks_per_voting_period = seconds_per_voting_period // spec.config.SECONDS_PER_ETH1_BLOCK
+    pow_blocks_to_merge = spec.TARGET_SECONDS_TO_MERGE // spec.config.SECONDS_PER_ETH1_BLOCK
+    pow_blocks_after_anchor_block = spec.config.ETH1_FOLLOW_DISTANCE + pow_blocks_per_voting_period + pow_blocks_to_merge
+    return spec.config.MIN_ANCHOR_POW_BLOCK_DIFFICULTY * uint256(pow_blocks_after_anchor_block)
+
+
+@with_merge_and_later
+@spec_state_test
+def test_zero_difficulty(spec, state):
+    anchor_pow_block = prepare_empty_pow_block(spec)
+    reference_td = compute_terminal_total_difficulty_reference(spec)
+
+    assert spec.compute_terminal_total_difficulty(anchor_pow_block) == reference_td