diff --git a/qa/rpc-tests/bip65-cltv-p2p.py b/qa/rpc-tests/bip65-cltv-p2p.py
new file mode 100755
index 00000000..1f8548c2
--- /dev/null
+++ b/qa/rpc-tests/bip65-cltv-p2p.py
@@ -0,0 +1,175 @@
+#!/usr/bin/env python2
+#
+# Distributed under the MIT/X11 software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+
+from test_framework.test_framework import ComparisonTestFramework
+from test_framework.util import *
+from test_framework.mininode import CTransaction, NetworkThread
+from test_framework.blocktools import create_coinbase, create_block
+from test_framework.comptool import TestInstance, TestManager
+from test_framework.script import CScript, OP_1NEGATE, OP_NOP2, OP_DROP
+from binascii import hexlify, unhexlify
+import cStringIO
+import time
+
+def cltv_invalidate(tx):
+    '''Modify the signature in vin 0 of the tx to fail CLTV
+
+    Prepends -1 CLTV DROP in the scriptSig itself.
+    '''
+    tx.vin[0].scriptSig = CScript([OP_1NEGATE, OP_NOP2, OP_DROP] +
+                                  list(CScript(tx.vin[0].scriptSig)))
+
+'''
+This test is meant to exercise BIP65 (CHECKLOCKTIMEVERIFY)
+Connect to a single node.
+Mine 2 (version 3) blocks (save the coinbases for later).
+Generate 98 more version 3 blocks, verify the node accepts.
+Mine 749 version 4 blocks, verify the node accepts.
+Check that the new CLTV rules are not enforced on the 750th version 4 block.
+Check that the new CLTV rules are enforced on the 751st version 4 block.
+Mine 199 new version blocks.
+Mine 1 old-version block.
+Mine 1 new version block.
+Mine 1 old version block, see that the node rejects.
+'''
+
+class BIP65Test(ComparisonTestFramework):
+
+    def __init__(self):
+        self.num_nodes = 1
+
+    def setup_network(self):
+        # Must set the blockversion for this test
+        self.nodes = start_nodes(1, self.options.tmpdir,
+                                 extra_args=[['-debug', '-whitelist=127.0.0.1', '-blockversion=3']],
+                                 binary=[self.options.testbinary])
+
+    def run_test(self):
+        test = TestManager(self, self.options.tmpdir)
+        test.add_all_connections(self.nodes)
+        NetworkThread().start() # Start up network handling in another thread
+        test.run()
+
+    def create_transaction(self, node, coinbase, to_address, amount):
+        from_txid = node.getblock(coinbase)['tx'][0]
+        inputs = [{ "txid" : from_txid, "vout" : 0}]
+        outputs = { to_address : amount }
+        rawtx = node.createrawtransaction(inputs, outputs)
+        signresult = node.signrawtransaction(rawtx)
+        tx = CTransaction()
+        f = cStringIO.StringIO(unhexlify(signresult['hex']))
+        tx.deserialize(f)
+        return tx
+
+    def get_tests(self):
+
+        self.coinbase_blocks = self.nodes[0].generate(2)
+        self.tip = int ("0x" + self.nodes[0].getbestblockhash() + "L", 0)
+        self.nodeaddress = self.nodes[0].getnewaddress()
+        self.last_block_time = time.time()
+
+        ''' 98 more version 3 blocks '''
+        test_blocks = []
+        for i in xrange(98):
+            block = create_block(self.tip, create_coinbase(2), self.last_block_time + 1)
+            block.nVersion = 3
+            block.rehash()
+            block.solve()
+            test_blocks.append([block, True])
+            self.last_block_time += 1
+            self.tip = block.sha256
+        yield TestInstance(test_blocks, sync_every_block=False)
+
+        ''' Mine 749 version 4 blocks '''
+        test_blocks = []
+        for i in xrange(749):
+            block = create_block(self.tip, create_coinbase(2), self.last_block_time + 1)
+            block.nVersion = 4
+            block.rehash()
+            block.solve()
+            test_blocks.append([block, True])
+            self.last_block_time += 1
+            self.tip = block.sha256
+        yield TestInstance(test_blocks, sync_every_block=False)
+
+        '''
+        Check that the new CLTV rules are not enforced in the 750th
+        version 3 block.
+        '''
+        spendtx = self.create_transaction(self.nodes[0],
+                self.coinbase_blocks[0], self.nodeaddress, 1.0)
+        cltv_invalidate(spendtx)
+        spendtx.rehash()
+
+        block = create_block(self.tip, create_coinbase(2), self.last_block_time + 1)
+        block.nVersion = 4
+        block.vtx.append(spendtx)
+        block.hashMerkleRoot = block.calc_merkle_root()
+        block.rehash()
+        block.solve()
+
+        self.last_block_time += 1
+        self.tip = block.sha256
+        yield TestInstance([[block, True]])
+
+        '''
+        Check that the new CLTV rules are enforced in the 751st version 4
+        block.
+        '''
+        spendtx = self.create_transaction(self.nodes[0],
+                self.coinbase_blocks[1], self.nodeaddress, 1.0)
+        cltv_invalidate(spendtx)
+        spendtx.rehash()
+
+        block = create_block(self.tip, create_coinbase(1), self.last_block_time + 1)
+        block.nVersion = 4
+        block.vtx.append(spendtx)
+        block.hashMerkleRoot = block.calc_merkle_root()
+        block.rehash()
+        block.solve()
+        self.last_block_time += 1
+        yield TestInstance([[block, False]])
+
+        ''' Mine 199 new version blocks on last valid tip '''
+        test_blocks = []
+        for i in xrange(199):
+            block = create_block(self.tip, create_coinbase(1), self.last_block_time + 1)
+            block.nVersion = 4
+            block.rehash()
+            block.solve()
+            test_blocks.append([block, True])
+            self.last_block_time += 1
+            self.tip = block.sha256
+        yield TestInstance(test_blocks, sync_every_block=False)
+
+        ''' Mine 1 old version block '''
+        block = create_block(self.tip, create_coinbase(1), self.last_block_time + 1)
+        block.nVersion = 3
+        block.rehash()
+        block.solve()
+        self.last_block_time += 1
+        self.tip = block.sha256
+        yield TestInstance([[block, True]])
+
+        ''' Mine 1 new version block '''
+        block = create_block(self.tip, create_coinbase(1), self.last_block_time + 1)
+        block.nVersion = 4
+        block.rehash()
+        block.solve()
+        self.last_block_time += 1
+        self.tip = block.sha256
+        yield TestInstance([[block, True]])
+
+        ''' Mine 1 old version block, should be invalid '''
+        block = create_block(self.tip, create_coinbase(1), self.last_block_time + 1)
+        block.nVersion = 3
+        block.rehash()
+        block.solve()
+        self.last_block_time += 1
+        yield TestInstance([[block, False]])
+
+if __name__ == '__main__':
+    BIP65Test().main()
diff --git a/qa/rpc-tests/bip65-cltv.py b/qa/rpc-tests/bip65-cltv.py
new file mode 100755
index 00000000..e60395dc
--- /dev/null
+++ b/qa/rpc-tests/bip65-cltv.py
@@ -0,0 +1,89 @@
+#!/usr/bin/env python2
+# Copyright (c) 2015 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test the CHECKLOCKTIMEVERIFY (BIP65) soft-fork logic
+#
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
+import os
+import shutil
+
+class BIP65Test(BitcoinTestFramework):
+
+    def setup_network(self):
+        self.nodes = []
+        self.nodes.append(start_node(0, self.options.tmpdir, []))
+        self.nodes.append(start_node(1, self.options.tmpdir, ["-blockversion=3"]))
+        self.nodes.append(start_node(2, self.options.tmpdir, ["-blockversion=4"]))
+        connect_nodes(self.nodes[1], 0)
+        connect_nodes(self.nodes[2], 0)
+        self.is_network_split = False
+        self.sync_all()
+
+    def run_test(self):
+        cnt = self.nodes[0].getblockcount()
+
+        # Mine some old-version blocks
+        self.nodes[1].generate(100)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 100):
+            raise AssertionError("Failed to mine 100 version=3 blocks")
+
+        # Mine 750 new-version blocks
+        for i in xrange(15):
+            self.nodes[2].generate(50)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 850):
+            raise AssertionError("Failed to mine 750 version=4 blocks")
+
+        # TODO: check that new CHECKLOCKTIMEVERIFY rules are not enforced
+
+        # Mine 1 new-version block
+        self.nodes[2].generate(1)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 851):
+            raise AssertionFailure("Failed to mine a version=4 blocks")
+
+        # TODO: check that new CHECKLOCKTIMEVERIFY rules are enforced
+
+        # Mine 198 new-version blocks
+        for i in xrange(2):
+            self.nodes[2].generate(99)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 1049):
+            raise AssertionError("Failed to mine 198 version=4 blocks")
+
+        # Mine 1 old-version block
+        self.nodes[1].generate(1)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 1050):
+            raise AssertionError("Failed to mine a version=3 block after 949 version=4 blocks")
+
+        # Mine 1 new-version blocks
+        self.nodes[2].generate(1)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 1051):
+            raise AssertionError("Failed to mine a version=3 block")
+
+        # Mine 1 old-version blocks
+        try:
+            self.nodes[1].generate(1)
+            raise AssertionError("Succeeded to mine a version=3 block after 950 version=4 blocks")
+        except JSONRPCException:
+            pass
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 1051):
+            raise AssertionError("Accepted a version=3 block after 950 version=4 blocks")
+
+        # Mine 1 new-version blocks
+        self.nodes[2].generate(1)
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 1052):
+            raise AssertionError("Failed to mine a version=4 block")
+
+if __name__ == '__main__':
+    BIP65Test().main()
diff --git a/src/consensus/consensus.h b/src/consensus/consensus.h
index 9c5b7d4f..f937844e 100644
--- a/src/consensus/consensus.h
+++ b/src/consensus/consensus.h
@@ -12,7 +12,5 @@ static const unsigned int MAX_BLOCK_SIZE = 1000000;
 static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** Coinbase transaction outputs can only be spent after this number of new blocks (network rule) */
 static const int COINBASE_MATURITY = 100;
-/** Threshold for nLockTime: below this value it is interpreted as block number, otherwise as UNIX timestamp. */
-static const unsigned int LOCKTIME_THRESHOLD = 500000000; // Tue Nov  5 00:53:20 1985 UTC
 
 #endif // BITCOIN_CONSENSUS_CONSENSUS_H
diff --git a/src/main.cpp b/src/main.cpp
index f6deaa29..e57fdb8c 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1853,11 +1853,18 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
 
     unsigned int flags = fStrictPayToScriptHash ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE;
 
-    // Start enforcing the DERSIG (BIP66) rules, for block.nVersion=3 blocks, when 75% of the network has upgraded:
+    // Start enforcing the DERSIG (BIP66) rules, for block.nVersion=3 blocks,
+    // when 75% of the network has upgraded:
     if (block.nVersion >= 3 && IsSuperMajority(3, pindex->pprev, chainparams.GetConsensus().nMajorityEnforceBlockUpgrade, chainparams.GetConsensus())) {
         flags |= SCRIPT_VERIFY_DERSIG;
     }
 
+    // Start enforcing CHECKLOCKTIMEVERIFY, (BIP65) for block.nVersion=4
+    // blocks, when 75% of the network has upgraded:
+    if (block.nVersion >= 4 && IsSuperMajority(4, pindex->pprev, chainparams.GetConsensus().nMajorityEnforceBlockUpgrade, chainparams.GetConsensus())) {
+        flags |= SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY;
+    }
+
     CBlockUndo blockundo;
 
     CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
@@ -2788,6 +2795,11 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
         return state.Invalid(error("%s : rejected nVersion=2 block", __func__),
                              REJECT_OBSOLETE, "bad-version");
 
+    // Reject block.nVersion=3 blocks when 95% (75% on testnet) of the network has upgraded:
+    if (block.nVersion < 4 && IsSuperMajority(4, pindexPrev, consensusParams.nMajorityRejectBlockOutdated, consensusParams))
+        return state.Invalid(error("%s : rejected nVersion=3 block", __func__),
+                             REJECT_OBSOLETE, "bad-version");
+
     return true;
 }
 
diff --git a/src/primitives/block.h b/src/primitives/block.h
index 59f46deb..90b8904a 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -21,7 +21,7 @@ class CBlockHeader
 {
 public:
     // header
-    static const int32_t CURRENT_VERSION=3;
+    static const int32_t CURRENT_VERSION=4;
     int32_t nVersion;
     uint256 hashPrevBlock;
     uint256 hashMerkleRoot;
diff --git a/src/rpcblockchain.cpp b/src/rpcblockchain.cpp
index 79528db2..1bab3b66 100644
--- a/src/rpcblockchain.cpp
+++ b/src/rpcblockchain.cpp
@@ -466,6 +466,36 @@ Value verifychain(const Array& params, bool fHelp)
     return CVerifyDB().VerifyDB(pcoinsTip, nCheckLevel, nCheckDepth);
 }
 
+/** Implementation of IsSuperMajority with better feedback */
+Object SoftForkMajorityDesc(int minVersion, CBlockIndex* pindex, int nRequired, const Consensus::Params& consensusParams)
+{
+    int nFound = 0;
+    CBlockIndex* pstart = pindex;
+    for (int i = 0; i < consensusParams.nMajorityWindow && pstart != NULL; i++)
+    {
+        if (pstart->nVersion >= minVersion)
+            ++nFound;
+        pstart = pstart->pprev;
+    }
+
+    Object rv;
+    rv.push_back(Pair("status", nFound >= nRequired));
+    rv.push_back(Pair("found", nFound));
+    rv.push_back(Pair("required", nRequired));
+    rv.push_back(Pair("window", consensusParams.nMajorityWindow));
+    return rv;
+}
+
+Object SoftForkDesc(const std::string &name, int version, CBlockIndex* pindex, const Consensus::Params& consensusParams)
+{
+    Object rv;
+    rv.push_back(Pair("id", name));
+    rv.push_back(Pair("version", version));
+    rv.push_back(Pair("enforce", SoftForkMajorityDesc(version, pindex, consensusParams.nMajorityEnforceBlockUpgrade, consensusParams)));
+    rv.push_back(Pair("reject", SoftForkMajorityDesc(version, pindex, consensusParams.nMajorityRejectBlockOutdated, consensusParams)));
+    return rv;
+}
+
 Value getblockchaininfo(const Array& params, bool fHelp)
 {
     if (fHelp || params.size() != 0)
@@ -481,6 +511,19 @@ Value getblockchaininfo(const Array& params, bool fHelp)
             "  \"difficulty\": xxxxxx,     (numeric) the current difficulty\n"
             "  \"verificationprogress\": xxxx, (numeric) estimate of verification progress [0..1]\n"
             "  \"chainwork\": \"xxxx\"     (string) total amount of work in active chain, in hexadecimal\n"
+            "  \"softforks\": [            (array) status of softforks in progress\n"
+            "     {\n"
+            "        \"id\": \"xxxx\",        (string) name of softfork\n"
+            "        \"version\": xx,         (numeric) block version\n"
+            "        \"enforce\": {           (object) progress toward enforcing the softfork rules for new-version blocks\n"
+            "           \"status\": xx,       (boolean) true if threshold reached\n"
+            "           \"found\": xx,        (numeric) number of blocks with the new version found\n"
+            "           \"required\": xx,     (numeric) number of blocks required to trigger\n"
+            "           \"window\": xx,       (numeric) maximum size of examined window of recent blocks\n"
+            "        },\n"
+            "        \"reject\": { ... }      (object) progress toward rejecting pre-softfork blocks (same fields as \"enforce\")\n"
+            "     }, ...\n"
+            "  ]\n"
             "}\n"
             "\nExamples:\n"
             + HelpExampleCli("getblockchaininfo", "")
@@ -498,6 +541,15 @@ Value getblockchaininfo(const Array& params, bool fHelp)
     obj.push_back(Pair("verificationprogress",  Checkpoints::GuessVerificationProgress(Params().Checkpoints(), chainActive.Tip())));
     obj.push_back(Pair("chainwork",             chainActive.Tip()->nChainWork.GetHex()));
     obj.push_back(Pair("pruned",                fPruneMode));
+
+    const Consensus::Params& consensusParams = Params().GetConsensus();
+    CBlockIndex* tip = chainActive.Tip();
+    Array softforks;
+    softforks.push_back(SoftForkDesc("bip34", 2, tip, consensusParams));
+    softforks.push_back(SoftForkDesc("bip66", 3, tip, consensusParams));
+    softforks.push_back(SoftForkDesc("bip65", 4, tip, consensusParams));
+    obj.push_back(Pair("softforks",             softforks));
+
     if (fPruneMode)
     {
         CBlockIndex *block = chainActive.Tip();
diff --git a/src/script/bitcoinconsensus.h b/src/script/bitcoinconsensus.h
index 03205777..a48ff1e1 100644
--- a/src/script/bitcoinconsensus.h
+++ b/src/script/bitcoinconsensus.h
@@ -44,9 +44,10 @@ typedef enum bitcoinconsensus_error_t
 /** Script verification flags */
 enum
 {
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_NONE      = 0,
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_P2SH      = (1U << 0), // evaluate P2SH (BIP16) subscripts
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_DERSIG    = (1U << 2), // enforce strict DER (BIP66) compliance
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_NONE                = 0,
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_P2SH                = (1U << 0), // evaluate P2SH (BIP16) subscripts
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_DERSIG              = (1U << 2), // enforce strict DER (BIP66) compliance
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9), // enable CHECKLOCKTIMEVERIFY (BIP65)
 };
 
 /// Returns 1 if the input nIn of the serialized transaction pointed to by
diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 84a7432f..0b78fdf5 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -335,9 +335,51 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 // Control
                 //
                 case OP_NOP:
-                break;
+                    break;
 
-                case OP_NOP1: case OP_NOP2: case OP_NOP3: case OP_NOP4: case OP_NOP5:
+                case OP_CHECKLOCKTIMEVERIFY:
+                {
+                    if (!(flags & SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY)) {
+                        // not enabled; treat as a NOP2
+                        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
+                            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
+                        }
+                        break;
+                    }
+
+                    if (stack.size() < 1)
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+
+                    // Note that elsewhere numeric opcodes are limited to
+                    // operands in the range -2**31+1 to 2**31-1, however it is
+                    // legal for opcodes to produce results exceeding that
+                    // range. This limitation is implemented by CScriptNum's
+                    // default 4-byte limit.
+                    //
+                    // If we kept to that limit we'd have a year 2038 problem,
+                    // even though the nLockTime field in transactions
+                    // themselves is uint32 which only becomes meaningless
+                    // after the year 2106.
+                    //
+                    // Thus as a special case we tell CScriptNum to accept up
+                    // to 5-byte bignums, which are good until 2**39-1, well
+                    // beyond the 2**32-1 limit of the nLockTime field itself.
+                    const CScriptNum nLockTime(stacktop(-1), fRequireMinimal, 5);
+
+                    // In the rare event that the argument may be < 0 due to
+                    // some arithmetic being done first, you can always use
+                    // 0 MAX CHECKLOCKTIMEVERIFY.
+                    if (nLockTime < 0)
+                        return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);
+
+                    // Actually compare the specified lock time with the transaction.
+                    if (!checker.CheckLockTime(nLockTime))
+                        return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);
+
+                    break;
+                }
+
+                case OP_NOP1: case OP_NOP3: case OP_NOP4: case OP_NOP5:
                 case OP_NOP6: case OP_NOP7: case OP_NOP8: case OP_NOP9: case OP_NOP10:
                 {
                     if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
@@ -1084,6 +1126,43 @@ bool TransactionSignatureChecker::CheckSig(const vector<unsigned char>& vchSigIn
     return true;
 }
 
+bool TransactionSignatureChecker::CheckLockTime(const CScriptNum& nLockTime) const
+{
+    // There are two times of nLockTime: lock-by-blockheight
+    // and lock-by-blocktime, distinguished by whether
+    // nLockTime < LOCKTIME_THRESHOLD.
+    //
+    // We want to compare apples to apples, so fail the script
+    // unless the type of nLockTime being tested is the same as
+    // the nLockTime in the transaction.
+    if (!(
+        (txTo->nLockTime <  LOCKTIME_THRESHOLD && nLockTime <  LOCKTIME_THRESHOLD) ||
+        (txTo->nLockTime >= LOCKTIME_THRESHOLD && nLockTime >= LOCKTIME_THRESHOLD)
+    ))
+        return false;
+
+    // Now that we know we're comparing apples-to-apples, the
+    // comparison is a simple numeric one.
+    if (nLockTime > (int64_t)txTo->nLockTime)
+        return false;
+
+    // Finally the nLockTime feature can be disabled and thus
+    // CHECKLOCKTIMEVERIFY bypassed if every txin has been
+    // finalized by setting nSequence to maxint. The
+    // transaction would be allowed into the blockchain, making
+    // the opcode ineffective.
+    //
+    // Testing if this vin is not final is sufficient to
+    // prevent this condition. Alternatively we could test all
+    // inputs, but testing just this input minimizes the data
+    // required to prove correct CHECKLOCKTIMEVERIFY execution.
+    if (txTo->vin[nIn].IsFinal())
+        return false;
+
+    return true;
+}
+
+
 bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
     set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index fc64438f..35d572f0 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -76,6 +76,11 @@ enum
     // (softfork safe, BIP62 rule 6)
     // Note: CLEANSTACK should never be used without P2SH.
     SCRIPT_VERIFY_CLEANSTACK = (1U << 8),
+
+    // Verify CHECKLOCKTIMEVERIFY
+    //
+    // See BIP65 for details.
+    SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9),
 };
 
 uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
@@ -88,6 +93,11 @@ public:
         return false;
     }
 
+    virtual bool CheckLockTime(const CScriptNum& nLockTime) const
+    {
+         return false;
+    }
+
     virtual ~BaseSignatureChecker() {}
 };
 
@@ -103,6 +113,7 @@ protected:
 public:
     TransactionSignatureChecker(const CTransaction* txToIn, unsigned int nInIn) : txTo(txToIn), nIn(nInIn) {}
     bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode) const;
+    bool CheckLockTime(const CScriptNum& nLockTime) const;
 };
 
 class MutableTransactionSignatureChecker : public TransactionSignatureChecker
diff --git a/src/script/script.h b/src/script/script.h
index d5045005..c09899aa 100644
--- a/src/script/script.h
+++ b/src/script/script.h
@@ -19,6 +19,10 @@
 
 static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520; // bytes
 
+// Threshold for nLockTime: below this value it is interpreted as block number,
+// otherwise as UNIX timestamp.
+static const unsigned int LOCKTIME_THRESHOLD = 500000000; // Tue Nov  5 00:53:20 1985 UTC
+
 template <typename T>
 std::vector<unsigned char> ToByteVector(const T& in)
 {
@@ -151,6 +155,7 @@ enum opcodetype
     // expansion
     OP_NOP1 = 0xb0,
     OP_NOP2 = 0xb1,
+    OP_CHECKLOCKTIMEVERIFY = OP_NOP2,
     OP_NOP3 = 0xb2,
     OP_NOP4 = 0xb3,
     OP_NOP5 = 0xb4,
@@ -196,7 +201,10 @@ public:
         m_value = n;
     }
 
-    explicit CScriptNum(const std::vector<unsigned char>& vch, bool fRequireMinimal)
+    static const size_t nDefaultMaxNumSize = 4;
+
+    explicit CScriptNum(const std::vector<unsigned char>& vch, bool fRequireMinimal,
+                        const size_t nMaxNumSize = nDefaultMaxNumSize)
     {
         if (vch.size() > nMaxNumSize) {
             throw scriptnum_error("script number overflow");
@@ -319,8 +327,6 @@ public:
         return result;
     }
 
-    static const size_t nMaxNumSize = 4;
-
 private:
     static int64_t set_vch(const std::vector<unsigned char>& vch)
     {
diff --git a/src/script/script_error.cpp b/src/script/script_error.cpp
index d8ecfde1..f1aa1fb4 100644
--- a/src/script/script_error.cpp
+++ b/src/script/script_error.cpp
@@ -47,6 +47,10 @@ const char* ScriptErrorString(const ScriptError serror)
             return "OP_RETURN was encountered";
         case SCRIPT_ERR_UNBALANCED_CONDITIONAL:
             return "Invalid OP_IF construction";
+        case SCRIPT_ERR_NEGATIVE_LOCKTIME:
+            return "Negative locktime";
+        case SCRIPT_ERR_UNSATISFIED_LOCKTIME:
+            return "Locktime requirement not satisfied";
         case SCRIPT_ERR_SIG_HASHTYPE:
             return "Signature hash type missing or not understood";
         case SCRIPT_ERR_SIG_DER:
diff --git a/src/script/script_error.h b/src/script/script_error.h
index 6365680b..bb10b8a2 100644
--- a/src/script/script_error.h
+++ b/src/script/script_error.h
@@ -35,6 +35,10 @@ typedef enum ScriptError_t
     SCRIPT_ERR_INVALID_ALTSTACK_OPERATION,
     SCRIPT_ERR_UNBALANCED_CONDITIONAL,
 
+    /* OP_CHECKLOCKTIMEVERIFY */
+    SCRIPT_ERR_NEGATIVE_LOCKTIME,
+    SCRIPT_ERR_UNSATISFIED_LOCKTIME,
+
     /* BIP62 */
     SCRIPT_ERR_SIG_HASHTYPE,
     SCRIPT_ERR_SIG_DER,
diff --git a/src/script/standard.h b/src/script/standard.h
index e4203f37..6d72bad2 100644
--- a/src/script/standard.h
+++ b/src/script/standard.h
@@ -51,6 +51,7 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_NULLDUMMY |
                                                          SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
                                                          SCRIPT_VERIFY_CLEANSTACK |
+                                                         SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
                                                          SCRIPT_VERIFY_LOW_S;
 
 /** For convenience, standard but not mandatory verify flags. */
diff --git a/src/test/data/tx_invalid.json b/src/test/data/tx_invalid.json
index 31d33c63..20bdbd08 100644
--- a/src/test/data/tx_invalid.json
+++ b/src/test/data/tx_invalid.json
@@ -120,6 +120,78 @@
 [[["a955032f4d6b0c9bfe8cad8f00a8933790b9c1dc28c82e0f48e75b35da0e4944", 0, "IF CODESEPARATOR ENDIF 0x21 0x0378d430274f8c5ec1321338151e9f27f4c676a008bdf8638d07c0b6be9ab35c71 CHECKSIGVERIFY CODESEPARATOR 1"]],
 "010000000144490eda355be7480f2ec828dcc1b9903793a8008fad8cfe9b0c6b4d2f0355a9000000004a483045022100fa4a74ba9fd59c59f46c3960cf90cbe0d2b743c471d24a3d5d6db6002af5eebb02204d70ec490fd0f7055a7c45f86514336e3a7f03503dacecabb247fc23f15c83510100ffffffff010000000000000000016a00000000", "P2SH"],
 
+["CHECKLOCKTIMEVERIFY tests"],
+
+["By-height locks, with argument just beyond tx nLockTime"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "1 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "499999999 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000fe64cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["By-time locks, with argument just beyond tx nLockTime (but within numerical boundries)"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "500000001 NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "4294967295 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000feffffff", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Argument missing"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000001b1010000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Argument negative with by-blockheight nLockTime=0"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "-1 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Argument negative with by-blocktime nLockTime=500,000,000"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "-1 NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000004005194b1010000000100000000000000000002000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Input locked"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000ffffffff0100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000251b1ffffffff0100000000000000000002000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Another input being unlocked isn't sufficient; the CHECKLOCKTIMEVERIFY-using input must be unlocked"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"] ,
+  ["0000000000000000000000000000000000000000000000000000000000000200", 1, "1"]],
+"010000000200010000000000000000000000000000000000000000000000000000000000000000000000ffffffff00020000000000000000000000000000000000000000000000000000000000000100000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Argument/tx height/time mismatch, both versions"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000251b100000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "499999999 NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "500000000 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "500000000 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ff64cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Argument 2^32 with nLockTime=2^32-1"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "4294967296 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ffffffff", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Same, but with nLockTime=2^31-1"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "2147483648 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ffffff7f", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["6 byte non-minimally-encoded arguments are invalid even in their contents are valid"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0x06 0x000000000000 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Failure due to failing CHECKLOCKTIMEVERIFY in scriptSig"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000251b1000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Failure due to failing CHECKLOCKTIMEVERIFY in redeemScript"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "HASH160 0x14 0xc5b93064159b3b2d6ab506a41b1f50463771b988 EQUAL"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000030251b1000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
 
 ["Make diffs cleaner by leaving a comment here without comma at the end"]
 ]
diff --git a/src/test/data/tx_valid.json b/src/test/data/tx_valid.json
index 182b88ef..24fff575 100644
--- a/src/test/data/tx_valid.json
+++ b/src/test/data/tx_valid.json
@@ -187,5 +187,47 @@
 "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", "P2SH"],
 
 
+["CHECKLOCKTIMEVERIFY tests"],
+
+["By-height locks, with argument == 0 and == tx nLockTime"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "499999999 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ff64cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ff64cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["By-time locks, with argument just beyond tx nLockTime (but within numerical boundries)"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "500000000 NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "4294967295 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ffffffff", "P2SH,CHECKLOCKTIMEVERIFY"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "500000000 NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000ffffffff", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Any non-maxint nSequence is fine"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000feffffff0100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["The argument can be calculated rather than created directly by a PUSHDATA"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "499999999 1ADD NOP2 1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000065cd1d", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Perhaps even by an ADD producing a 5-byte result that is out of bounds for other opcodes"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "2147483647 2147483647 ADD NOP2 1"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000feffffff", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["5 byte non-minimally-encoded arguments are valid"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "0x05 0x0000000000 NOP2 1"]],
+"010000000100010000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Valid CHECKLOCKTIMEVERIFY in scriptSig"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "1"]],
+"01000000010001000000000000000000000000000000000000000000000000000000000000000000000251b1000000000100000000000000000001000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
+["Valid CHECKLOCKTIMEVERIFY in redeemScript"],
+[[["0000000000000000000000000000000000000000000000000000000000000100", 0, "HASH160 0x14 0xc5b93064159b3b2d6ab506a41b1f50463771b988 EQUAL"]],
+"0100000001000100000000000000000000000000000000000000000000000000000000000000000000030251b1000000000100000000000000000001000000", "P2SH,CHECKLOCKTIMEVERIFY"],
+
 ["Make diffs cleaner by leaving a comment here without comma at the end"]
 ]
diff --git a/src/test/scriptnum_tests.cpp b/src/test/scriptnum_tests.cpp
index 24c7dd3d..d95724db 100644
--- a/src/test/scriptnum_tests.cpp
+++ b/src/test/scriptnum_tests.cpp
@@ -145,7 +145,7 @@ static void RunCreate(const int64_t& num)
 {
     CheckCreateInt(num);
     CScriptNum scriptnum(num);
-    if (scriptnum.getvch().size() <= CScriptNum::nMaxNumSize)
+    if (scriptnum.getvch().size() <= CScriptNum::nDefaultMaxNumSize)
         CheckCreateVch(num);
     else
     {
diff --git a/src/test/transaction_tests.cpp b/src/test/transaction_tests.cpp
index 4439814e..28152c64 100644
--- a/src/test/transaction_tests.cpp
+++ b/src/test/transaction_tests.cpp
@@ -40,7 +40,8 @@ static std::map<string, unsigned int> mapFlagNames = boost::assign::map_list_of
     (string("MINIMALDATA"), (unsigned int)SCRIPT_VERIFY_MINIMALDATA)
     (string("NULLDUMMY"), (unsigned int)SCRIPT_VERIFY_NULLDUMMY)
     (string("DISCOURAGE_UPGRADABLE_NOPS"), (unsigned int)SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
-    (string("CLEANSTACK"), (unsigned int)SCRIPT_VERIFY_CLEANSTACK);
+    (string("CLEANSTACK"), (unsigned int)SCRIPT_VERIFY_CLEANSTACK)
+    (string("CHECKLOCKTIMEVERIFY"), (unsigned int)SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY);
 
 unsigned int ParseScriptFlags(string strFlags)
 {