diff --git a/main.py b/main.py
index 383b5fb..17d3fbd 100644
--- a/main.py
+++ b/main.py
@@ -19,6 +19,8 @@
 import argparse
 import asyncio
 import logging
+import os
+import re
 import sys
 
 from nacl.signing import SigningKey
@@ -271,11 +273,27 @@ async def cli_loop(sk, pk, chain, mempool, network):
 # Main entry point
 # ──────────────────────────────────────────────
 
-async def run_node(port: int, connect_to: str | None, fund: int):
+async def run_node(port: int, connect_to: str | None, fund: int, datadir: str | None):
     """Boot the node, optionally connect to a peer, then enter the CLI."""
     sk, pk = create_wallet()
 
-    chain = Blockchain()
+    # Load existing chain from disk, or start fresh
+    chain = None
+    if datadir and os.path.exists(os.path.join(datadir, "data.json")):
+        try:
+            from minichain.persistence import load
+            chain = load(datadir)
+            logger.info("Restored chain from '%s'", datadir)
+        except FileNotFoundError as e:
+            logger.warning("Could not load saved chain: %s — starting fresh", e)
+        except ValueError as e:
+            logger.error("State data is corrupted or tampered: %s", e)
+            logger.error("Refusing to start to avoid overwriting corrupted data.")
+            sys.exit(1)
+
+    if chain is None:
+        chain = Blockchain()
+
     mempool = Mempool()
     network = P2PNetwork()
 
@@ -313,6 +331,14 @@ async def on_peer_connected(writer):
     try:
         await cli_loop(sk, pk, chain, mempool, network)
     finally:
+        # Save chain to disk on shutdown
+        if datadir:
+            try:
+                from minichain.persistence import save
+                save(chain, datadir)
+                logger.info("Chain saved to '%s'", datadir)
+            except Exception as e:
+                logger.error("Failed to save chain during shutdown: %s", e)
         await network.stop()
 
 
@@ -321,6 +347,7 @@ def main():
     parser.add_argument("--port", type=int, default=9000, help="TCP port to listen on (default: 9000)")
     parser.add_argument("--connect", type=str, default=None, help="Peer address to connect to (host:port)")
     parser.add_argument("--fund", type=int, default=100, help="Initial coins to fund this wallet (default: 100)")
+    parser.add_argument("--datadir", type=str, default=None, help="Directory to save/load blockchain state (enables persistence)")
     args = parser.parse_args()
 
     logging.basicConfig(
@@ -330,7 +357,7 @@ def main():
     )
 
     try:
-        asyncio.run(run_node(args.port, args.connect, args.fund))
+        asyncio.run(run_node(args.port, args.connect, args.fund, args.datadir))
     except KeyboardInterrupt:
         print("\nNode shut down.")
 
diff --git a/minichain/__init__.py b/minichain/__init__.py
index a3e42ae..ae52604 100644
--- a/minichain/__init__.py
+++ b/minichain/__init__.py
@@ -6,6 +6,7 @@
 from .contract import ContractMachine
 from .p2p import P2PNetwork
 from .mempool import Mempool
+from .persistence import save, load
 
 __all__ = [
     "mine_block",
@@ -18,4 +19,6 @@
     "ContractMachine",
     "P2PNetwork",
     "Mempool",
+    "save",
+    "load",
 ]
diff --git a/minichain/persistence.py b/minichain/persistence.py
new file mode 100644
index 0000000..367a41d
--- /dev/null
+++ b/minichain/persistence.py
@@ -0,0 +1,218 @@
+"""
+Chain persistence: save and load the blockchain and state to/from JSON.
+
+Design:
+  - blockchain.json  holds the full list of serialised blocks
+  - state.json       holds the accounts dict (includes off-chain credits)
+
+Both files are written atomically (temp → rename) to prevent corruption
+on crash.  On load, chain integrity is verified before the data is trusted.
+
+Usage:
+    from minichain.persistence import save, load
+
+    save(blockchain, path="data/")
+    blockchain = load(path="data/")
+"""
+
+import json
+import os
+import tempfile
+import logging
+import copy
+
+from .block import Block
+from .transaction import Transaction
+from .chain import Blockchain
+from .state import State
+from .pow import calculate_hash
+
+logger = logging.getLogger(__name__)
+
+_DATA_FILE = "data.json"
+
+
+# ---------------------------------------------------------------------------
+# Public API
+# ---------------------------------------------------------------------------
+
+def save(blockchain: Blockchain, path: str = ".") -> None:
+    """
+    Persist the blockchain and account state to a JSON file inside *path*.
+
+    Uses atomic write (write-to-temp → rename) with fsync so a crash mid-save
+    never corrupts the existing file. Chain and state are saved together to
+    prevent torn snapshots.
+    """
+    os.makedirs(path, exist_ok=True)
+
+    with blockchain._lock:  # Thread-safe: hold lock while serialising
+        chain_data = [block.to_dict() for block in blockchain.chain]
+        state_data = copy.deepcopy(blockchain.state.accounts)
+
+    snapshot = {
+        "chain": chain_data,
+        "state": state_data
+    }
+
+    _atomic_write_json(os.path.join(path, _DATA_FILE), snapshot)
+
+    logger.info(
+        "Saved %d blocks and %d accounts to '%s'",
+        len(chain_data),
+        len(state_data),
+        path,
+    )
+
+
+def load(path: str = ".") -> Blockchain:
+    """
+    Restore a Blockchain from the JSON file inside *path*.
+
+    Steps:
+      1. Load and deserialise blocks from data.json
+      2. Verify chain integrity (genesis, linkage, hashes)
+      3. Load account state
+
+    Raises:
+        FileNotFoundError: if data.json is missing.
+        ValueError:        if data is invalid or integrity checks fail.
+    """
+    data_path = os.path.join(path, _DATA_FILE)
+    snapshot = _read_json(data_path)
+
+    if not isinstance(snapshot, dict):
+        raise ValueError(f"Invalid snapshot data in '{data_path}'")
+
+    raw_blocks = snapshot.get("chain")
+    raw_accounts = snapshot.get("state")
+
+    if not isinstance(raw_blocks, list) or not raw_blocks:
+        raise ValueError(f"Invalid or empty chain data in '{data_path}'")
+    if not isinstance(raw_accounts, dict):
+        raise ValueError(f"Invalid accounts data in '{data_path}'")
+
+    blocks = [_deserialize_block(b) for b in raw_blocks]
+
+    # --- Integrity verification ---
+    _verify_chain_integrity(blocks)
+
+    # --- Rebuild blockchain properly (no __new__ hack) ---
+    blockchain = Blockchain()           # creates genesis + fresh state
+    blockchain.chain = blocks           # replace with loaded chain
+
+    # Restore state
+    blockchain.state.accounts = raw_accounts
+
+    logger.info(
+        "Loaded %d blocks and %d accounts from '%s'",
+        len(blockchain.chain),
+        len(blockchain.state.accounts),
+        path,
+    )
+    return blockchain
+
+
+# ---------------------------------------------------------------------------
+# Integrity verification
+# ---------------------------------------------------------------------------
+
+def _verify_chain_integrity(blocks: list) -> None:
+    """Verify genesis, hash linkage, and block hashes."""
+    # Check genesis
+    genesis = blocks[0]
+    if genesis.index != 0 or genesis.hash != "0" * 64:
+        raise ValueError("Invalid genesis block")
+
+    # Check linkage and hashes for every subsequent block
+    for i in range(1, len(blocks)):
+        block = blocks[i]
+        prev = blocks[i - 1]
+
+        if block.index != prev.index + 1:
+            raise ValueError(
+                f"Block #{block.index}: index gap (expected {prev.index + 1})"
+            )
+
+        if block.previous_hash != prev.hash:
+            raise ValueError(
+                f"Block #{block.index}: previous_hash mismatch"
+            )
+
+        expected_hash = calculate_hash(block.to_header_dict())
+        if block.hash != expected_hash:
+            raise ValueError(
+                f"Block #{block.index}: hash mismatch "
+                f"(stored={block.hash[:16]}..., computed={expected_hash[:16]}...)"
+            )
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def _atomic_write_json(filepath: str, data) -> None:
+    """Write JSON atomically with fsync for durability."""
+    dir_name = os.path.dirname(filepath) or "."
+    fd, tmp_path = tempfile.mkstemp(dir=dir_name, suffix=".tmp")
+    try:
+        with os.fdopen(fd, "w", encoding="utf-8") as f:
+            json.dump(data, f, indent=2)
+            f.flush()
+            os.fsync(f.fileno())  # Ensure data is on disk
+        os.replace(tmp_path, filepath)   # Atomic rename
+
+        # Attempt to fsync the directory so the rename is durable
+        if hasattr(os, "O_DIRECTORY"):
+            try:
+                dir_fd = os.open(dir_name, os.O_RDONLY | os.O_DIRECTORY)
+                try:
+                    os.fsync(dir_fd)
+                finally:
+                    os.close(dir_fd)
+            except OSError:
+                pass  # Directory fsync not supported on all platforms
+
+    except BaseException:
+        try:
+            os.unlink(tmp_path)
+        except OSError:
+            pass
+        raise
+
+
+def _read_json(filepath: str):
+    if not os.path.exists(filepath):
+        raise FileNotFoundError(f"Persistence file not found: '{filepath}'")
+    with open(filepath, "r", encoding="utf-8") as f:
+        return json.load(f)
+
+
+def _deserialize_block(data: dict) -> Block:
+    """Reconstruct a Block (including its transactions) from a plain dict."""
+    transactions = [
+        Transaction(
+            sender=tx["sender"],
+            receiver=tx["receiver"],
+            amount=tx["amount"],
+            nonce=tx["nonce"],
+            data=tx.get("data"),
+            signature=tx.get("signature"),
+            timestamp=tx["timestamp"],
+        )
+        for tx in data.get("transactions", [])
+    ]
+
+    block = Block(
+        index=data["index"],
+        previous_hash=data["previous_hash"],
+        transactions=transactions,
+        timestamp=data["timestamp"],
+        difficulty=data.get("difficulty"),
+    )
+    block.nonce = data["nonce"]
+    block.hash = data["hash"]
+    # Only overwrite merkle_root if explicitly saved; otherwise keep computed value
+    if "merkle_root" in data:
+        block.merkle_root = data["merkle_root"]
+    return block
diff --git a/tests/test_persistence.py b/tests/test_persistence.py
new file mode 100644
index 0000000..e758227
--- /dev/null
+++ b/tests/test_persistence.py
@@ -0,0 +1,201 @@
+"""
+Tests for chain persistence (save / load round-trip).
+"""
+
+import json
+import os
+import shutil
+import tempfile
+import unittest
+
+from nacl.signing import SigningKey
+from nacl.encoding import HexEncoder
+
+from minichain import Blockchain, Transaction, Block, mine_block
+from minichain.persistence import save, load
+
+
+def _make_keypair():
+    sk = SigningKey.generate()
+    pk = sk.verify_key.encode(encoder=HexEncoder).decode()
+    return sk, pk
+
+
+class TestPersistence(unittest.TestCase):
+
+    def setUp(self):
+        self.tmpdir = tempfile.mkdtemp()
+
+    def tearDown(self):
+        shutil.rmtree(self.tmpdir, ignore_errors=True)
+
+    # Helpers
+
+    def _chain_with_tx(self):
+        """Return a Blockchain that has one mined block with a transfer."""
+        bc = Blockchain()
+        alice_sk, alice_pk = _make_keypair()
+        _, bob_pk = _make_keypair()
+
+        bc.state.credit_mining_reward(alice_pk, 100)
+
+        tx = Transaction(alice_pk, bob_pk, 30, 0)
+        tx.sign(alice_sk)
+
+        block = Block(
+            index=1,
+            previous_hash=bc.last_block.hash,
+            transactions=[tx],
+            difficulty=1,
+        )
+        mine_block(block, difficulty=1)
+        bc.add_block(block)
+        return bc, alice_pk, bob_pk
+
+    # --- Basic save/load ---
+
+    def test_save_creates_file(self):
+        bc = Blockchain()
+        save(bc, path=self.tmpdir)
+        self.assertTrue(os.path.exists(os.path.join(self.tmpdir, "data.json")))
+
+    def test_chain_length_preserved(self):
+        bc, _, _ = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        restored = load(path=self.tmpdir)
+        self.assertEqual(len(restored.chain), len(bc.chain))
+
+    def test_block_hashes_preserved(self):
+        bc, _, _ = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        restored = load(path=self.tmpdir)
+        for original, loaded in zip(bc.chain, restored.chain):
+            self.assertEqual(original.hash, loaded.hash)
+            self.assertEqual(original.index, loaded.index)
+            self.assertEqual(original.previous_hash, loaded.previous_hash)
+
+    def test_transaction_data_preserved(self):
+        bc, _, _ = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        restored = load(path=self.tmpdir)
+        original_tx = bc.chain[1].transactions[0]
+        loaded_tx = restored.chain[1].transactions[0]
+
+        self.assertEqual(original_tx.sender, loaded_tx.sender)
+        self.assertEqual(original_tx.receiver, loaded_tx.receiver)
+        self.assertEqual(original_tx.amount, loaded_tx.amount)
+        self.assertEqual(original_tx.nonce, loaded_tx.nonce)
+        self.assertEqual(original_tx.signature, loaded_tx.signature)
+
+    def test_genesis_only_chain(self):
+        bc = Blockchain()
+        save(bc, path=self.tmpdir)
+        restored = load(path=self.tmpdir)
+
+        self.assertEqual(len(restored.chain), 1)
+        self.assertEqual(restored.chain[0].hash, "0" * 64)
+
+    # --- State recomputation ---
+
+    def test_state_recomputed_from_blocks(self):
+        """Balances must be recomputed by replaying blocks, not from a file."""
+        bc, alice_pk, bob_pk = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        restored = load(path=self.tmpdir)
+        # Alice started with 100, sent 30 → 70
+        self.assertEqual(
+            restored.state.get_account(alice_pk)["balance"],
+            bc.state.get_account(alice_pk)["balance"],
+        )
+        # Bob received 30
+        self.assertEqual(
+            restored.state.get_account(bob_pk)["balance"],
+            bc.state.get_account(bob_pk)["balance"],
+        )
+
+    # --- Integrity verification ---
+
+    def test_tampered_hash_rejected(self):
+        """Loading a chain with a tampered block hash must raise ValueError."""
+        bc, _, _ = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        # Tamper with block hash
+        chain_path = os.path.join(self.tmpdir, "data.json")
+        with open(chain_path, "r") as f:
+            data = json.load(f)
+        data["chain"][1]["hash"] = "deadbeef" * 8
+        with open(chain_path, "w") as f:
+            json.dump(data, f)
+
+        with self.assertRaises(ValueError):
+            load(path=self.tmpdir)
+
+    def test_broken_linkage_rejected(self):
+        """Loading a chain with broken previous_hash linkage must raise."""
+        bc, _, _ = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        chain_path = os.path.join(self.tmpdir, "data.json")
+        with open(chain_path, "r") as f:
+            data = json.load(f)
+        data["chain"][1]["previous_hash"] = "0" * 64 + "ff"
+        with open(chain_path, "w") as f:
+            json.dump(data, f)
+
+        with self.assertRaises(ValueError):
+            load(path=self.tmpdir)
+
+    # --- Crash safety ---
+
+    def test_corrupted_json_raises(self):
+        """Half-written JSON must raise an error, not silently corrupt."""
+        bc = Blockchain()
+        save(bc, path=self.tmpdir)
+
+        # Corrupt the file
+        chain_path = os.path.join(self.tmpdir, "data.json")
+        with open(chain_path, "w") as f:
+            f.write('{"truncated": ')  # invalid JSON
+
+        with self.assertRaises(json.JSONDecodeError):
+            load(path=self.tmpdir)
+
+    def test_missing_file_raises(self):
+        with self.assertRaises(FileNotFoundError):
+            load(path=self.tmpdir)  # nothing saved yet
+
+    # --- Chain continuity after load ---
+
+    def test_loaded_chain_can_add_new_block(self):
+        """Restored chain must still accept new valid blocks."""
+        bc, alice_pk, bob_pk = self._chain_with_tx()
+        save(bc, path=self.tmpdir)
+
+        restored = load(path=self.tmpdir)
+
+        # Build a second transfer using a new key
+        new_sk, new_pk = _make_keypair()
+        restored.state.credit_mining_reward(new_pk, 50)
+
+        tx2 = Transaction(new_pk, bob_pk, 10, 0)
+        tx2.sign(new_sk)
+
+        block2 = Block(
+            index=len(restored.chain),
+            previous_hash=restored.last_block.hash,
+            transactions=[tx2],
+            difficulty=1,
+        )
+        mine_block(block2, difficulty=1)
+
+        self.assertTrue(restored.add_block(block2))
+        self.assertEqual(len(restored.chain), len(bc.chain) + 1)
+
+
+if __name__ == "__main__":
+    unittest.main()