diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index 40435029..ecafb236 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -190,7 +190,75 @@ Block.prototype._buildMerkle = function buildMerkle() {
   return merkleTree;
 };
 
+// This mimics the behavior of CheckBlockHeader()
+// and CheckBlock() in bitcoin/src/main.cpp.
 Block.prototype._checkBlock = function checkBlock() {
+  // Check proof of work matches claimed amount
+  if (!utils.testTarget(this.bits, this.hash())) {
+    return false;
+  }
+
+  // Check timestamp
+  if (+new Date(this.ts) > Date.now() + 2 * 60 * 60 * 1000) {
+    return false;
+  }
+
+  // Size of all txs cant be bigger than MAX_BLOCK_SIZE
+  if (this.txs.length > bcoin.protocol.constants.block.maxSize) {
+    return false;
+  }
+
+  // First TX must be a coinbase
+  if (this.txs[0].inputs.length !== 1 || +this.txs[0].inputs[0].out.hash !== 0) {
+    return false;
+  }
+
+  // The rest of the txs must not be coinbases
+  for (var 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 transactions
+  // TODO:
+  // for (var i = 0; i < this.txs.length; i++) {
+  //   if (!this.txs[i].verify()) {
+  //     return false;
+  //   }
+  // }
+
+  // Build MerkleTree
   this.merkleTree = this._buildMerkle();
+
+  // Check for duplicate tx ids
+  var unique = [];
+  for (var i = 0; i < this.txs.length; i++) {
+    if (!~unique.indexOf(this.txs[i].hash('hex'))) {
+      unique.push(this.txs[i].hash('hex'));
+    }
+  }
+  if (unique.length !== this.txs.length) {
+    return false;
+  }
+
+  // Check to make sure block does not have more opcodes than  MAX_BLOCK_SIGOPS
+  var sigops = 0;
+  // TODO:
+  // for (var i = 0; this.txs.length; i++) {
+  //   for (var j = 0; j < this.txs.inputs.length; j++) {
+  //     var script = this.txs.inputs[j].script;
+  //     sigops += countSigops(script);
+  //   }
+  //   for (var j = 0; j < this.txs.outputs.length; j++) {
+  //     var script = this.txs.outputs[j].script;
+  //     sigops += countSigops(script);
+  //   }
+  // }
+  if (sigops > bcoin.protocol.constants.block.maxSigops) {
+    return false;
+  }
+
+  // Check merkle root
   return this.merkleTree[this.merkleTree.length-1] === this.merkleRoot;
 };
diff --git a/lib/bcoin/protocol/constants.js b/lib/bcoin/protocol/constants.js
index a3be0e7b..0842b379 100644
--- a/lib/bcoin/protocol/constants.js
+++ b/lib/bcoin/protocol/constants.js
@@ -147,3 +147,9 @@ exports.hashType = {
   single: 3,
   anyonecaypay: 0x80
 };
+
+exports.block = {
+  maxSize: 1000000,
+  maxSigops: 1000000 / 50,
+  maxOrphanTx: 1000000 / 100
+};