diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index 148ce031..684c39b1 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -42,6 +42,8 @@ function Block(data, subtype) {
   this.network = data.network || false;
   this.relayedBy = data.relayedBy || '0.0.0.0';
 
+  this._chain = data.chain;
+
   this.valid = null;
   this._hash = null;
 
@@ -226,7 +228,8 @@ Block.prototype.getMerkleRoot = function getMerkleRoot() {
 // This mimics the behavior of CheckBlockHeader()
 // and CheckBlock() in bitcoin/src/main.cpp.
 Block.prototype._verify = function _verify() {
-  var i, unique, hash, merkleRoot;
+  var uniq = {};
+  var i, tx, hash;
 
   // Check proof of work matches claimed amount
   if (!utils.testTarget(this.bits, this.hash()))
@@ -251,55 +254,106 @@ Block.prototype._verify = function _verify() {
   }
 
   // First TX must be a coinbase
-  if (!this.txs.length
-      || this.txs[0].inputs.length !== 1
-      || +this.txs[0].inputs[0].out.hash !== 0)
+  if (!this.txs.length || !this.txs[0].isCoinbase())
     return false;
 
-  // The rest of the txs must not be coinbases
-  for (i = 1; i < this.txs.length; i++) {
-    if (this.txs[i].inputs.length === 1
-        && +this.txs[i].inputs[0].out.hash === 0)
-      return false;
-  }
-
-  // Check for duplicate tx ids
-  unique = {};
+  // Test all txs
   for (i = 0; i < this.txs.length; i++) {
-    hash = this.txs[i].hash('hex');
-    if (unique[hash])
-      return false;
-    unique[hash] = true;
-  }
+    tx = this.txs[i];
 
-  // Build MerkleTree
-  merkleRoot = this.getMerkleRoot();
+    // The rest of the txs must not be coinbases
+    if (i > 0 && tx.isCoinbase())
+      return false;
+
+    // Check for duplicate txids
+    hash = tx.hash('hex');
+    if (uniq[hash])
+      return false;
+    uniq[hash] = true;
+  }
 
   // Check merkle root
-  if (merkleRoot !== this.merkleRoot)
+  if (this.getMerkleRoot() !== this.merkleRoot)
     return false;
 
   return true;
 };
 
-Block.prototype.getHeight = function getHeight(chain) {
-  chain = chain || bcoin.chain.global;
+Block.prototype.postVerify = function postVerify() {
+  var prev, i, tx, cb;
 
-  if (!chain)
-    return -1;
+  if (this.subtype !== 'block')
+    return true;
 
-  return chain.getHeight(this.hash('hex'));
+  if (!this.chain)
+    return true;
+
+  prev = this.chain.getBlock(this.prevBlock);
+
+  // Ensure it's not an orphan
+  if (!prev)
+    return false;
+
+  // Ensure the timestamp is correct
+  if (this.ts <= prev.getMedianTime())
+    return false;
+
+  // Test all txs
+  for (i = 0; i < this.txs.length; i++) {
+    tx = this.txs[i];
+
+    // TXs must be finalized with regards to seq and locktime
+    if (!tx.isFinal(this, prev))
+      return false;
+  }
+
+  if (this.bits !== this.chain.target(prev, this))
+    return false;
+
+  if (this.version < 2 && prev.isOutdated(2))
+    return false;
+
+  if (this.version < 3 && prev.isOutdated(3))
+    return false;
+
+  if (this.version < 4 && prev.isOutdated(4))
+    return false;
+
+  // Enforce height in coinbase
+  if (this.version >= 2 && prev.needsUpgrade(2)) {
+    cb = bcoin.script.isCoinbase(this.txs[0].inputs[0].script, this);
+
+    if (!cb)
+      return false;
+
+    if (cb.height !== prev.height + 1)
+      return false;
+  }
+
+  // sig validation (bip66)
+  if (this.version >= 3 && prev.needsUpgrade(3))
+    this.scriptFlags |= 1; // dersig
+
+  // checklocktimeverify (bip65)
+  if (this.version >= 4 && prev.needsUpgrade(4))
+    this.scriptFlags |= 2;
+
+  return true;
 };
 
-Block.prototype.getNextBlock = function getNextBlock(chain) {
+Block.prototype.getHeight = function getHeight() {
+  if (!this.chain)
+    return -1;
+  return this.chain.getHeight(this.hash('hex'));
+};
+
+Block.prototype.getNextBlock = function getNextBlock() {
   var next;
 
-  chain = chain || bcoin.chain.global;
-
-  if (!chain)
+  if (!this.chain)
     return utils.toHex(constants.zeroHash);
 
-  next = chain.getNextBlock(this.hash('hex'));
+  next = this.chain.getNextBlock(this.hash('hex'));
 
   if (!next)
     return utils.toHex(constants.zeroHash);
@@ -361,26 +415,32 @@ Block.prototype.getReward = function getReward() {
   };
 };
 
-Block.prototype.getEntry = function getEntry(chain) {
-  chain = chain || bcoin.chain.global;
-  return chain.getBlock(this);
+Block.prototype.getEntry = function getEntry() {
+  if (!this.chain)
+    return;
+  return this.chain.getBlock(this);
 };
 
-Block.prototype.isOrphan = function isOrphan(chain) {
-  chain = chain || bcoin.chain.global;
-  return chain.hasBlock(this.prevBlock);
+Block.prototype.isOrphan = function isOrphan() {
+  if (!this.chain)
+    return true;
+  return this.chain.hasBlock(this.prevBlock);
 };
 
+Block.prototype.__defineGetter__('chain', function() {
+  return this._chain || bcoin.chain.global;
+});
+
 Block.prototype.__defineGetter__('rhash', function() {
   return utils.revHex(this.hash('hex'));
 });
 
 Block.prototype.__defineGetter__('height', function() {
-  return this.getHeight(bcoin.chain.global);
+  return this.getHeight();
 });
 
 Block.prototype.__defineGetter__('nextBlock', function() {
-  return this.getNextBlock(bcoin.chain.global);
+  return this.getNextBlock();
 });
 
 Block.prototype.__defineGetter__('reward', function() {
@@ -399,17 +459,18 @@ Block.prototype.__defineGetter__('coinbase', function() {
 });
 
 Block.prototype.__defineGetter__('entry', function() {
-  return this.getEntry(bcoin.chain.global);
+  return this.getEntry();
 });
 
 Block.prototype.__defineGetter__('orphan', function() {
-  return this.isOrphan(bcoin.chain.global);
+  return this.isOrphan();
 });
 
 Block.prototype.inspect = function inspect() {
   var copy = bcoin.block(this, this.subtype);
   copy.__proto__ = null;
   delete copy._raw;
+  delete copy._chain;
   copy.hash = this.hash('hex');
   copy.rhash = this.rhash;
   copy.height = this.height;
diff --git a/lib/bcoin/chain.js b/lib/bcoin/chain.js
index fb942284..ce7f2208 100644
--- a/lib/bcoin/chain.js
+++ b/lib/bcoin/chain.js
@@ -149,7 +149,7 @@ Chain.prototype._addIndex = function _addIndex(entry) {
     return Chain.codes.unchanged;
   }
 
-  // Duplcate height
+  // Duplicate height
   if (this.index.hashes[entry.height] === entry.hash)
     return Chain.codes.unchanged;
 
@@ -294,8 +294,16 @@ Chain.prototype.add = function add(block, peer) {
       // If we have a block at the same height, use chain with higher work
       if (this.index.hashes[entry.height]) {
         if (this.tip.chainwork.cmp(entry.chainwork) < 0) {
+          if (!block.postVerify()) {
+            throw new Error;
+            //code = Chain.codes.invalid;
+            //break;
+          }
           this.resetHeight(entry.height - 1);
-          this._addIndex(entry);
+          code = this._addIndex(entry);
+          assert(code !== Chain.codes.unchanged);
+          if (code !== Chain.codes.okay)
+            break;
           code = Chain.codes.forked;
           // Breaking here only works because
           // we deleted the orphan map in resetHeight.
@@ -308,7 +316,15 @@ Chain.prototype.add = function add(block, peer) {
       }
 
       // Validated known block at this point - add it to index
+      if (!block.postVerify()) {
+        throw new Error;
+        //code = Chain.codes.invalid;
+        //break;
+      }
       code = this._addIndex(entry);
+      assert(code !== Chain.codes.unchanged);
+      if (code !== Chain.codes.okay)
+        break;
       this.emit('block', block, peer);
       this.emit('entry', entry);
       if (block !== initial)
@@ -531,26 +547,24 @@ Chain.prototype.height = function height() {
 };
 
 Chain.prototype.target = function target(last, block) {
-  var proofOfWorkLimit = utils.toCompact(network.powLimit);
-  var adjustmentInterval = network.powTargetTimespan / network.powTargetSpacing;
-  var newBlockTs, heightFirst, first;
-
-  adjustmentInterval |= 0;
+  var powLimit = utils.toCompact(network.powLimit);
+  var interval = network.powTargetTimespan / network.powTargetSpacing | 0;
+  var first, ts;
 
   if (!last)
     last = this.getTip();
 
   // Do not retarget
-  if ((last.height + 1) % adjustmentInterval) {
+  if ((last.height + 1) % interval) {
     if (network.powAllowMinDifficultyBlocks) {
       // Special behavior for testnet:
-      newBlockTs = block ? block.ts : utils.now();
-      if (newBlockTs > last.ts + network.powTargetSpacing * 2)
-        return proofOfWorkLimit;
+      ts = block ? (block.ts || block) : utils.now();
+      if (ts > last.ts + network.powTargetSpacing * 2)
+        return powLimit;
 
       while (last.prev
-        && last.height % adjustmentInterval !== 0
-        && last.bits !== proofOfWorkLimit) {
+        && last.height % interval !== 0
+        && last.bits !== powLimit) {
         last = last.prev;
       }
 
@@ -560,23 +574,22 @@ Chain.prototype.target = function target(last, block) {
   }
 
   // Back 2 weeks
-  heightFirst = last.height - (adjustmentInterval - 1);
-  first = this.byHeight(heightFirst);
+  first = this.byHeight(last.height - (interval - 1));
 
   if (!first)
     return 0;
 
-  return this.retarget(last, first.ts);
+  return this.retarget(last, first);
 };
 
-Chain.prototype.retarget = function retarget(last, firstTs) {
+Chain.prototype.retarget = function retarget(last, first) {
   var powTargetTimespan = new bn(network.powTargetTimespan);
   var actualTimespan, powLimit, target;
 
   if (network.powNoRetargeting)
     return last.bits;
 
-  actualTimespan = new bn(last.ts).subn(firstTs);
+  actualTimespan = new bn(last.ts).subn(first.ts);
   if (actualTimespan.cmp(powTargetTimespan.divn(4)) < 0)
     actualTimespan = powTargetTimespan.divn(4);
 
@@ -752,6 +765,45 @@ ChainBlock.prototype.getChainwork = function() {
   return (this.prev ? this.prev.chainwork : new bn(0)).add(this.proof);
 };
 
+ChainBlock.prototype.getMedianTime = function() {
+  var entry = this;
+  var median = [];
+  var timeSpan = constants.block.medianTimespan;
+  var i;
+
+  for (i = 0; i < timeSpan && entry; i++, entry = entry.prev)
+    median.push(entry.ts);
+
+  median = median.sort();
+
+  return median[median.length / 2 | 0];
+};
+
+ChainBlock.prototype.isOutdated = function(version) {
+  return this.isSuperMajority(version,
+    network.block.majorityRejectBlockOutdated);
+};
+
+ChainBlock.prototype.needsUpgrade = function(version) {
+  return this.isSuperMajority(version,
+    network.block.majorityEnforceBlockUpgrade);
+};
+
+ChainBlock.prototype.isSuperMajority = function(version, required) {
+  var entry = this;
+  var found = 0;
+  var majorityWindow = network.block.majorityWindow;
+  var i;
+
+  for (i = 0; i < majorityWindow && found < required && entry; i++) {
+    if (entry.version >= version)
+      found++;
+    entry = entry.prev;
+  }
+
+  return found >= required;
+};
+
 ChainBlock.prototype.toJSON = function() {
   // return [
   //   this.hash,
diff --git a/lib/bcoin/miner.js b/lib/bcoin/miner.js
index b27086d3..8bea3ba7 100644
--- a/lib/bcoin/miner.js
+++ b/lib/bcoin/miner.js
@@ -134,12 +134,17 @@ Miner.prototype.addTX = function addTX(tx) {
       return;
   }
 
-  // Pretty important
+  // Ignore if it's already in a block
+  if (tx.height !== -1)
+    return;
+
   if (!tx.verify())
     return;
 
-  // Ignore if it's already in a block
-  if (tx.height !== -1)
+  if (tx.isCoinbase())
+    return;
+
+  if (!tx.isFinal(this.block, this.last))
     return;
 
   // Deliver me from the block size debate, please
@@ -161,10 +166,12 @@ Miner.prototype.addTX = function addTX(tx) {
 };
 
 Miner.prototype.createBlock = function createBlock(tx) {
-  var target, coinbase, headers, block;
+  var ts, target, coinbase, headers, block;
 
-  // Update target
-  target = this.chain.target(this.last);
+  ts = Math.max(utils.now(), this.last.ts + 1);
+
+  // Find target
+  target = this.chain.target(this.last, ts);
 
   // Create a coinbase
   coinbase = bcoin.tx();
@@ -194,7 +201,7 @@ Miner.prototype.createBlock = function createBlock(tx) {
       ? this.last.hash('hex')
       : this.last.hash,
     merkleRoot: utils.toHex(constants.zeroHash.slice()),
-    ts: utils.now(),
+    ts: ts,
     bits: utils.toCompact(target),
     nonce: 0
   };
diff --git a/lib/bcoin/peer.js b/lib/bcoin/peer.js
index e7296264..a5fc2133 100644
--- a/lib/bcoin/peer.js
+++ b/lib/bcoin/peer.js
@@ -149,8 +149,9 @@ Peer.prototype._init = function init() {
 
   this._req('verack', function(err, payload) {
     if (err) {
+      self._error(err);
       self.destroy();
-      return self._error(err);
+      return;
     }
     self.ack = true;
     self.emit('ack');
@@ -318,7 +319,7 @@ Peer.prototype._res = function _res(cmd, payload) {
   for (i = 0; i < this._request.queue.length; i++) {
     entry = this._request.queue[i];
 
-    if (!entry || entry.cmd && entry.cmd !== cmd)
+    if (!entry || (entry.cmd && entry.cmd !== cmd))
       return false;
 
     res = entry.cb(null, payload, cmd);
diff --git a/lib/bcoin/pool.js b/lib/bcoin/pool.js
index 94eeba5c..89896eea 100644
--- a/lib/bcoin/pool.js
+++ b/lib/bcoin/pool.js
@@ -43,7 +43,9 @@ function Pool(options) {
   this.size = options.size || 32;
   this.parallel = options.parallel || 2000;
   this.redundancy = options.redundancy || 2;
-  this.seeds = network.seeds.slice();
+  this.seeds = options.seeds
+    ? options.seeds.slice()
+    : network.seeds.slice();
 
   this._createConnection = options.createConnection;
   this._createSocket = options.createSocket;
@@ -280,6 +282,10 @@ Pool.prototype._addLoader = function _addLoader() {
     relay: this.options.relay
   });
 
+  peer.once('socket', function() {
+    self.emit('debug', 'Added loader peer: %s', peer.host);
+  });
+
   this.peers.load = peer;
   this.peers.all.push(peer);
 
@@ -494,8 +500,10 @@ Pool.prototype._handleBlock = function _handleBlock(block, peer) {
     return;
 
   // Make sure the block is valid
-  if (!block.verify())
+  if (!block.verify()) {
+    this.emit('debug', 'Block verification failed for %s', block.hash('hex'));
     return;
+  }
 
   // Resolve orphan chain
   if (!this.options.headers) {
@@ -768,8 +776,15 @@ Pool.prototype._removePeer = function _removePeer(peer) {
   if (i !== -1)
     this.peers.all.splice(i, 1);
 
-  if (this.peers.load === peer)
+  if (this.peers.load === peer) {
+    this.emit('debug', 'Removed loader peer (%s).', peer.host);
     this.peers.load = null;
+    // i = this.seeds.indexOf(peer.host);
+    // if (i === -1)
+    //   i = this.seeds.indexOf(peer.host + ':' + peer.port);
+    // if (i !== -1)
+    //   this.seeds.splice(i, 1);
+  }
 };
 
 Pool.prototype.watch = function watch(id) {
@@ -1407,9 +1422,15 @@ Pool.prototype.usableSeed = function usableSeed(addrs, force) {
   if (!addrs)
     addrs = this.seeds;
 
-  for (i = 0; i < addrs.length; i++) {
-    if (!this.getPeer(addrs[i]))
-      return addrs[i];
+  addrs = addrs.slice().sort(function() {
+    return Math.random() > 0.50 ? 1 : -1;
+  });
+
+  if (this.peers.loader) {
+    for (i = 0; i < addrs.length; i++) {
+      if (!this.getPeer(addrs[i]))
+        return addrs[i];
+    }
   }
 
   if (!force)
diff --git a/lib/bcoin/protocol/constants.js b/lib/bcoin/protocol/constants.js
index 8b1e2320..2717d4ca 100644
--- a/lib/bcoin/protocol/constants.js
+++ b/lib/bcoin/protocol/constants.js
@@ -155,7 +155,8 @@ exports.hashTypeByVal = Object.keys(exports.hashType).reduce(function(out, type)
 exports.block = {
   maxSize: 1000000,
   maxSigops: 1000000 / 50,
-  maxOrphanTx: 1000000 / 100
+  maxOrphanTx: 1000000 / 100,
+  medianTimeSpan: 11
 };
 
 exports.script = {
diff --git a/lib/bcoin/protocol/network.js b/lib/bcoin/protocol/network.js
index 3a0c926b..81246dc8 100644
--- a/lib/bcoin/protocol/network.js
+++ b/lib/bcoin/protocol/network.js
@@ -140,6 +140,12 @@ main.powTargetSpacing = 10 * 60;
 main.powAllowMinDifficultyBlocks = false;
 main.powNoRetargeting = false;
 
+main.block = {
+  majorityEnforceBlockUpgrade: 750,
+  majorityRejectBlockOutdated: 950,
+  majorityWindow: 1000
+};
+
 /**
  * Testnet (v3)
  * https://en.bitcoin.it/wiki/Testnet
@@ -237,3 +243,9 @@ testnet.powTargetTimespan = 14 * 24 * 60 * 60; // two weeks
 testnet.powTargetSpacing = 10 * 60;
 testnet.powAllowMinDifficultyBlocks = true;
 testnet.powNoRetargeting = false;
+
+testnet.block = {
+  majorityEnforceBlockUpgrade: 51,
+  majorityRejectBlockOutdated: 75,
+  majorityWindow: 100
+};
diff --git a/lib/bcoin/tx.js b/lib/bcoin/tx.js
index 7a993df7..a6c658f5 100644
--- a/lib/bcoin/tx.js
+++ b/lib/bcoin/tx.js
@@ -37,6 +37,8 @@ function TX(data, block) {
   this.network = data.network || false;
   this.relayedBy = data.relayedBy || '0.0.0.0';
 
+  this._chain = data.chain;
+
   this._lock = this.lock;
 
   if (data.inputs) {
@@ -834,25 +836,66 @@ TX.prototype.funds = function funds(side) {
   return acc;
 };
 
-TX.prototype.getHeight = function getHeight(chain) {
-  chain = chain || bcoin.chain.global;
-
-  if (!chain)
-    return -1;
-
-  return this.block ? chain.getHeight(this.block) : -1;
+// Used for postVerify/ContextualBlockCheck and miner isFinalTx call.
+// BIP113 will require that time-locked transactions have nLockTime set to
+// less than the median time of the previous block they're contained in.
+TX.prototype.isFinal = function isFinal(block, prev) {
+  var height = prev.height + 1;
+  var ts = this.locktimeMedian ? prev.getMedianTime() : block.ts;
+  return this._isFinal(height, ts);
 };
 
-TX.prototype.getConfirmations = function getConfirmations(chain) {
+// Used in AcceptToMemoryPool
+TX.prototype.isFinalAccept = function isFinalAccept() {
+  var height = this.chain.height() + 1;
+  var ts = this.lockTimeMedian
+    ? this.chain.getTip().getMedianTime()
+    : utils.now();
+  return this._isFinalTx(height, ts);
+};
+
+// Used in the original bitcoind code for AcceptBlock
+TX.prototype._isFinalOriginal = function _isFinalOriginal(block) {
+  var ts = block ? block.ts : utils.now();
+  var height = this.chain.height();
+  return this._isFinalTx(height, ts);
+};
+
+TX.prototype._isFinal = function _isFinal(height, ts) {
+  var threshold = constants.locktimeThreshold;
+  var i;
+
+  if (!this.chain)
+    return true;
+
+  if (this.lock === 0)
+    return true;
+
+  if (this.lock < (this.lock < threshold ? height : ts))
+    return true;
+
+  for (i = 0; i < this.inputs.length; i++) {
+    if (this.inputs[i].seq !== 0xffffffff)
+      return false;
+  }
+
+  return true;
+};
+
+TX.prototype.getHeight = function getHeight() {
+  if (!this.chain)
+    return -1;
+  return this.block ? this.chain.getHeight(this.block) : -1;
+};
+
+TX.prototype.getConfirmations = function getConfirmations() {
   var top, height;
 
-  chain = chain || bcoin.chain.global;
-
-  if (!chain)
+  if (!this.chain)
     return 0;
 
-  top = chain.height();
-  height = this.getHeight(chain);
+  top = this.chain.height();
+  height = this.getHeight();
 
   if (height === -1)
     return 0;
@@ -860,6 +903,10 @@ TX.prototype.getConfirmations = function getConfirmations(chain) {
   return top - height + 1;
 };
 
+TX.prototype.__defineGetter__('chain', function() {
+  return this._chain || bcoin.chain.global;
+});
+
 TX.prototype.__defineGetter__('rblock', function() {
   return this.block
     ? utils.revHex(this.block)
@@ -879,11 +926,11 @@ TX.prototype.__defineGetter__('value', function() {
 });
 
 TX.prototype.__defineGetter__('height', function() {
-  return this.getHeight(bcoin.chain.global);
+  return this.getHeight();
 });
 
 TX.prototype.__defineGetter__('confirmations', function() {
-  return this.getConfirmations(bcoin.chain.global);
+  return this.getConfirmations();
 });
 
 TX.prototype.inspect = function inspect() {
@@ -892,6 +939,7 @@ TX.prototype.inspect = function inspect() {
   if (this.block)
     copy.block = this.block;
   delete copy._raw;
+  delete copy._chain;
   copy.hash = this.hash('hex');
   copy.rhash = this.rhash;
   copy.rblock = this.rblock;