diff --git a/pyproject.toml b/pyproject.toml
index 405a772e..62067402 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,7 +19,9 @@ classifiers = [
 ]
 requires-python = ">=3.12"
 dependencies = [
+    "ethereum-types<=0.2.4,<0.3",
     "pydantic>=2.9.2,<3",
+    "ssz>=0.5.2,<0.6",
     "typing-extensions>=4.4",
 ]
 
diff --git a/src/lean_spec/client/__init__.py b/src/lean_spec/client/__init__.py
new file mode 100644
index 00000000..604cd663
--- /dev/null
+++ b/src/lean_spec/client/__init__.py
@@ -0,0 +1,9 @@
+"""
+Specification for pqdevnet-0
+
+Key differences from 3SF-mini.py:
+- Using `U64` instead of native `int` for all fields
+- Using `Bytes32` instead of native `str` for all fields
+- Combined `*_root` and `*_slot` pairs into a single `Checkpoint` field
+- Removed optionals from `Block`
+"""
diff --git a/src/lean_spec/client/block.py b/src/lean_spec/client/block.py
new file mode 100644
index 00000000..01200346
--- /dev/null
+++ b/src/lean_spec/client/block.py
@@ -0,0 +1,27 @@
+"""
+A `Block` is a single link in the Lean Consensus chain. Each `Block` contains
+associated metadata like the slot number, parent block hash and votes.
+
+Together, these blocks form a cryptographically secure journal recording the
+history of all state transitions that have happened since the genesis of the
+chain.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+from pydantic import BaseModel, ConfigDict
+from ssz.sedes.list import List
+
+from .preset import VALIDATOR_REGISTRY_LIMIT
+from .vote import Vote
+
+
+class Block(BaseModel):
+    """A single block in the Lean Consensus chain."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    slot: U64
+    parent: Bytes32
+    votes: List[Vote, VALIDATOR_REGISTRY_LIMIT]
+    state_root: Bytes32
diff --git a/src/lean_spec/client/checkpoint.py b/src/lean_spec/client/checkpoint.py
new file mode 100644
index 00000000..6af0a8f5
--- /dev/null
+++ b/src/lean_spec/client/checkpoint.py
@@ -0,0 +1,17 @@
+"""
+A `Checkpoint` is a single checkpoint for a block in the Lean Consensus chain.
+Each `Checkpoint` contains its associated block root and slot.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+from pydantic import BaseModel, ConfigDict
+
+
+class Checkpoint(BaseModel):
+    """A single checkpoint in the Lean Consensus chain."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    root: Bytes32
+    slot: U64
diff --git a/src/lean_spec/client/preset.py b/src/lean_spec/client/preset.py
new file mode 100644
index 00000000..ed30399f
--- /dev/null
+++ b/src/lean_spec/client/preset.py
@@ -0,0 +1,26 @@
+"""
+The `preset` module contains the parameters that are used to configure the
+Lean Consensus chain.
+"""
+
+from ethereum_types.numeric import U64
+
+# Time parameters
+# ---------------------------------------------------------------
+
+# 4 seconds
+SLOT_DURATION_MS: U64 = U64(4000)
+
+# Basis points (out of 10000)
+PROPOSER_REORG_CUTOFF_BPS: U64 = U64(2500)
+VOTE_DUE_BPS: U64 = U64(5000)
+FAST_CONFIRM_DUE_BPS: U64 = U64(7500)
+VIEW_FREEZE_CUTOFF_BPS: U64 = U64(7500)
+
+# Misc
+# ---------------------------------------------------------------
+
+# 2^18, enough for 2^18 / (60 / 4) / 60 / 24 = 12.1 days
+MAX_HISTORICAL_BLOCK_HASHES: U64 = U64(262144)
+
+VALIDATOR_REGISTRY_LIMIT: U64 = U64(4096)
diff --git a/src/lean_spec/client/state.py b/src/lean_spec/client/state.py
new file mode 100644
index 00000000..6618cfda
--- /dev/null
+++ b/src/lean_spec/client/state.py
@@ -0,0 +1,37 @@
+"""
+A `State` is a collection of metadata that describes the current state of the
+Lean Consensus chain. It contains information about the latest justified and
+finalized blocks, as well as the historical block hashes and justified slots.
+
+It is used to verify the integrity of the chain and to ensure that the chain is
+progressing correctly.
+"""
+
+from ethereum_types.bytes import Bytes32
+from pydantic import BaseModel, ConfigDict
+from ssz.sedes.bitlist import Bitlist
+from ssz.sedes.list import List
+
+from .checkpoint import Checkpoint
+from .preset import MAX_HISTORICAL_BLOCK_HASHES, VALIDATOR_REGISTRY_LIMIT
+
+
+class State(BaseModel):
+    """Represents the current state of the Lean Consensus chain."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    # Diverged from 3SF-mini.py: Removed `config: Config` from the state
+
+    latest_justified: Checkpoint
+    latest_finalized: Checkpoint
+
+    historical_block_hashes: List[Bytes32, MAX_HISTORICAL_BLOCK_HASHES]
+    justified_slots: List[bool, MAX_HISTORICAL_BLOCK_HASHES]
+
+    # Diverged from 3SF-mini.py:
+    # Flattened `justifications: Dict[str, List[bool]]` for SSZ
+    justifications_roots: List[Bytes32, MAX_HISTORICAL_BLOCK_HASHES]
+    justifications_validators: Bitlist[
+        MAX_HISTORICAL_BLOCK_HASHES * VALIDATOR_REGISTRY_LIMIT
+    ]
diff --git a/src/lean_spec/client/vote.py b/src/lean_spec/client/vote.py
new file mode 100644
index 00000000..061d29ae
--- /dev/null
+++ b/src/lean_spec/client/vote.py
@@ -0,0 +1,22 @@
+"""
+A `Vote` is a single vote for a block in the Lean Consensus chain. Each `Vote`
+contains information about the validator that voted, the slot of the block they
+voted for, and the block hash they voted for.
+"""
+
+from ethereum_types.numeric import U64
+from pydantic import BaseModel, ConfigDict
+
+from .checkpoint import Checkpoint
+
+
+class Vote(BaseModel):
+    """A single vote for a block in the Lean Consensus chain."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    validator_id: U64
+    slot: U64
+    head: Checkpoint
+    target: Checkpoint
+    source: Checkpoint
diff --git a/tests/lean_spec/client/test_block.py b/tests/lean_spec/client/test_block.py
new file mode 100644
index 00000000..913a8c98
--- /dev/null
+++ b/tests/lean_spec/client/test_block.py
@@ -0,0 +1,39 @@
+"""
+Tests for the client's Block container.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+from ssz.sedes.list import List
+
+from lean_spec.client.block import Block
+from lean_spec.client.checkpoint import Checkpoint
+from lean_spec.client.preset import VALIDATOR_REGISTRY_LIMIT
+from lean_spec.client.vote import Vote
+
+
+def test_block():
+    Block(
+        slot=U64(1),
+        parent=Bytes32(b"\x02" * 32),
+        votes=List(
+            Vote(
+                validator_id=U64(1),
+                slot=U64(2),
+                head=Checkpoint(
+                    root=Bytes32(b"\x04" * 32),
+                    slot=U64(3),
+                ),
+                source=Checkpoint(
+                    root=Bytes32(b"\x05" * 32),
+                    slot=U64(5),
+                ),
+                target=Checkpoint(
+                    root=Bytes32(b"\x06" * 32),
+                    slot=U64(4),
+                ),
+            ),
+            int(VALIDATOR_REGISTRY_LIMIT),
+        ),
+        state_root=Bytes32(b"\x03" * 32),
+    )
diff --git a/tests/lean_spec/client/test_checkpoint.py b/tests/lean_spec/client/test_checkpoint.py
new file mode 100644
index 00000000..e786bb3b
--- /dev/null
+++ b/tests/lean_spec/client/test_checkpoint.py
@@ -0,0 +1,15 @@
+"""
+Tests for the client's Checkpoint container.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+
+from lean_spec.client.checkpoint import Checkpoint
+
+
+def test_checkpoint():
+    Checkpoint(
+        root=Bytes32(b"\x42" * 32),
+        slot=U64(42),
+    )
diff --git a/tests/lean_spec/client/test_state.py b/tests/lean_spec/client/test_state.py
new file mode 100644
index 00000000..fc2a4ba9
--- /dev/null
+++ b/tests/lean_spec/client/test_state.py
@@ -0,0 +1,52 @@
+"""
+Tests for the client's State container.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+from ssz.sedes.bitlist import Bitlist
+from ssz.sedes.list import List
+
+from lean_spec.client.checkpoint import Checkpoint
+from lean_spec.client.preset import (
+    MAX_HISTORICAL_BLOCK_HASHES,
+    VALIDATOR_REGISTRY_LIMIT,
+)
+from lean_spec.client.state import State
+
+
+def test_state():
+    State(
+        latest_justified=Checkpoint(
+            root=Bytes32(b"\x00" * 32),
+            slot=U64(0),
+        ),
+        latest_finalized=Checkpoint(
+            root=Bytes32(b"\x00" * 32),
+            slot=U64(0),
+        ),
+        historical_block_hashes=List(
+            [
+                Bytes32(b"\x00" * 32),
+                Bytes32(b"\x01" * 32),
+            ],
+            int(MAX_HISTORICAL_BLOCK_HASHES),
+        ),
+        justified_slots=List(
+            [
+                True,
+                False,
+            ],
+            int(MAX_HISTORICAL_BLOCK_HASHES),
+        ),
+        justifications_roots=List(
+            [
+                Bytes32(b"\x00" * 32),
+                Bytes32(b"\x01" * 32),
+            ],
+            int(MAX_HISTORICAL_BLOCK_HASHES),
+        ),
+        justifications_validators=Bitlist(
+            int(MAX_HISTORICAL_BLOCK_HASHES * VALIDATOR_REGISTRY_LIMIT)
+        ),
+    )
diff --git a/tests/lean_spec/client/test_vote.py b/tests/lean_spec/client/test_vote.py
new file mode 100644
index 00000000..b857073e
--- /dev/null
+++ b/tests/lean_spec/client/test_vote.py
@@ -0,0 +1,28 @@
+"""
+Tests for the client's Vote container.
+"""
+
+from ethereum_types.bytes import Bytes32
+from ethereum_types.numeric import U64
+
+from lean_spec.client.checkpoint import Checkpoint
+from lean_spec.client.vote import Vote
+
+
+def test_vote():
+    Vote(
+        validator_id=U64(1),
+        slot=U64(2),
+        head=Checkpoint(
+            root=Bytes32(b"\x03" * 32),
+            slot=U64(3),
+        ),
+        source=Checkpoint(
+            root=Bytes32(b"\x05" * 32),
+            slot=U64(5),
+        ),
+        target=Checkpoint(
+            root=Bytes32(b"\x04" * 32),
+            slot=U64(4),
+        ),
+    )