diff --git a/src/transaction_builder.js b/src/transaction_builder.js
new file mode 100644
index 0000000..e284d63
--- /dev/null
+++ b/src/transaction_builder.js
@@ -0,0 +1,159 @@
+var assert = require('assert')
+var scripts = require('./scripts')
+
+var ECKey = require('./eckey')
+var Transaction = require('./transaction')
+var Script = require('./script')
+
+function TransactionBuilder() {
+  this.prevOutMap = {}
+  this.prevOutScripts = {}
+  this.prevOutTypes = {}
+
+  this.signatures = []
+  this.tx = new Transaction()
+}
+
+TransactionBuilder.prototype.addInput = function(prevTx, index, prevOutScript) {
+  var prevOutHash
+
+  if (typeof prevTx === 'string') {
+    prevOutHash = new Buffer(prevTx, 'hex')
+
+    // TxId hex is big-endian, we want little-endian hash
+    Array.prototype.reverse.call(prevOutHash)
+
+  } else if (prevTx instanceof Transaction) {
+    assert(prevOutScript === undefined, 'Unnecessary Script provided')
+
+    prevOutHash = prevTx.getHash()
+    prevOutScript = prevTx.outs[index].script
+
+  } else {
+    prevOutHash = prevTx
+
+  }
+
+  var prevOutType
+  if (prevOutScript !== undefined) {
+    prevOutType = scripts.classifyOutput(prevOutScript)
+
+    assert.notEqual(prevOutType, 'nonstandard', 'PrevOutScript not supported (nonstandard)')
+  }
+
+  assert(this.signatures.every(function(input) {
+    return input.hashType & Transaction.SIGHASH_ANYONECANPAY
+  }), 'No, this would invalidate signatures')
+
+  var prevOut = prevOutHash.toString('hex') + ':' + index
+  assert(!(prevOut in this.prevOutMap), 'Transaction is already an input')
+
+  var vout = this.tx.addInput(prevOutHash, index)
+  this.prevOutMap[prevOut] = true
+  this.prevOutScripts[vout] = prevOutScript
+  this.prevOutTypes[vout] = prevOutType
+
+  return vout
+}
+
+TransactionBuilder.prototype.addOutput = function(scriptPubKey, value) {
+  assert(this.signatures.every(function(signature) {
+    return (signature.hashType & 0x1f) === Transaction.SIGHASH_SINGLE
+  }), 'No, this would invalidate signatures')
+
+  return this.tx.addOutput(scriptPubKey, value)
+}
+
+TransactionBuilder.prototype.sign = function(index, privKey, redeemScript, hashType) {
+  assert(this.tx.ins.length >= index, 'No input at index: ' + index)
+  hashType = hashType || Transaction.SIGHASH_ALL
+
+  var prevOutScript = this.prevOutScripts[index]
+  var prevOutType = this.prevOutTypes[index]
+
+  var scriptType, hash
+  if (redeemScript) {
+    prevOutScript = prevOutScript || scripts.scriptHashOutput(redeemScript.getHash())
+    prevOutType = prevOutType || 'scripthash'
+
+    assert.equal(prevOutType, 'scripthash', 'PrevOutScript must be P2SH')
+
+    scriptType = scripts.classifyOutput(redeemScript)
+
+    assert.notEqual(scriptType, 'scripthash', 'RedeemScript can\'t be P2SH')
+    assert.notEqual(scriptType, 'nonstandard', 'RedeemScript not supported (nonstandard)')
+
+    hash = this.tx.hashForSignature(index, redeemScript, hashType)
+
+  } else {
+    prevOutScript = prevOutScript || privKey.pub.getAddress().toOutputScript()
+    scriptType = prevOutType || 'pubkeyhash'
+
+    assert.notEqual(scriptType, 'scripthash', 'PrevOutScript requires redeemScript')
+
+    hash = this.tx.hashForSignature(index, prevOutScript, hashType)
+  }
+
+  var signature = privKey.sign(hash)
+
+  if (!(index in this.signatures)) {
+    this.signatures[index] = {
+      hashType: hashType,
+      pubKeys: [],
+      redeemScript: redeemScript,
+      scriptType: scriptType,
+      signatures: []
+    }
+  }
+
+  var input = this.signatures[index]
+  input.pubKeys.push(privKey.pub)
+  input.signatures.push(signature)
+
+  assert.equal(input.hashType, hashType, 'Inconsistent hashType')
+  assert.deepEqual(input.redeemScript, redeemScript, 'Inconsistent redeemScript')
+}
+
+TransactionBuilder.prototype.build = function(allowIncomplete) {
+  if (!allowIncomplete) {
+    assert(this.tx.ins.length > 0, 'Transaction has no inputs')
+    assert(this.tx.outs.length > 0, 'Transaction has no outputs')
+    assert.equal(this.signatures.length, this.tx.ins.length, 'Transaction is missing signatures')
+  }
+
+  var tx = this.tx.clone()
+
+  this.signatures.forEach(function(input, index) {
+    var scriptSig
+
+    if (input.scriptType === 'pubkeyhash') {
+      var signature = input.signatures[0].toScriptSignature(input.hashType)
+      var publicKey = input.pubKeys[0]
+      scriptSig = scripts.pubKeyHashInput(signature, publicKey)
+
+    } else if (input.scriptType === 'multisig') {
+      var redeemScript = allowIncomplete ? undefined : input.redeemScript
+      var signatures = input.signatures.map(function(signature) {
+        return signature.toScriptSignature(input.hashType)
+      })
+      scriptSig = scripts.multisigInput(signatures, redeemScript)
+
+    } else if (input.scriptType === 'pubkey') {
+      var signature = input.signatures[0]
+      scriptSig = scripts.pubKeyInput(signature)
+
+    } else {
+      assert(false, input.scriptType + ' not supported')
+    }
+
+    if (input.redeemScript) {
+      scriptSig = scripts.scriptHashInput(scriptSig, input.redeemScript)
+    }
+
+    tx.setInputScript(index, scriptSig)
+  })
+
+  return tx
+}
+
+module.exports = TransactionBuilder
diff --git a/test/fixtures/transaction_builder.json b/test/fixtures/transaction_builder.json
new file mode 100644
index 0000000..69c1fea
--- /dev/null
+++ b/test/fixtures/transaction_builder.json
@@ -0,0 +1,113 @@
+{
+  "valid": {
+    "build": [
+      {
+        "description": "pubKeyHash 1:1 transaction",
+        "txid": "bd641f4b0aa8bd70189ab45e935c4762f0e1c49f294b4779d79887937b7cf42e",
+        "inputs": [
+          {
+            "index": 0,
+            "prevTx": "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+            "privKeys": ["KwDiBf89QgGbjEhKnhXJuH7LrciVrZi3qYjgd9M7rFU73sVHnoWn"]
+          }
+        ],
+        "outputs": [
+          {
+            "script": "OP_DUP OP_HASH160 aa4d7985c57e011a8b3dd8e0e5a73aaef41629c5 OP_EQUALVERIFY OP_CHECKSIG",
+            "value": 10000
+          }
+        ]
+      },
+      {
+        "description": "2-of-2 P2SH multisig Transaction",
+        "txid": "8c500ce6eef6c78a10de923b68394cf31120151bdc4600e4b12de865defa9d24",
+        "inputs": [
+          {
+            "index": 0,
+            "prevTx": "4971f016798a167331bcbc67248313fbc444c6e92e4416efd06964425588f5cf",
+            "privKeys": ["91avARGdfge8E4tZfYLoxeJ5sGBdNJQH4kvjJoQFacbgwmaKkrx", "91avARGdfge8E4tZfYLoxeJ5sGBdNJQH4kvjJoQFacbgww7vXtT"],
+            "redeemScript": "OP_2 0479be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8 04c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee51ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52a OP_2 OP_CHECKMULTISIG"
+          }
+        ],
+        "outputs": [
+          {
+            "script": "OP_DUP OP_HASH160 faf1d99bf040ea9c7f8cc9f14ac6733ad75ce246 OP_EQUALVERIFY OP_CHECKSIG",
+            "value": 10000
+          }
+        ]
+      }
+    ]
+  },
+  "invalid": {
+    "build": [
+      {
+        "exception": "Transaction has no inputs",
+        "hex": "",
+        "outputs": [
+          {
+            "script": "",
+            "value": 1000
+          }
+        ]
+      },
+      {
+        "exception": "Transaction has no outputs",
+        "hex": "",
+        "inputs": [
+          {
+            "hash": "",
+            "index": 0
+          }
+        ]
+      },
+      {
+        "exception": "Transaction has no signatures",
+        "hex": "",
+        "inputs": [
+          {
+            "hash": "",
+            "index": 0
+          }
+        ],
+        "outputs": [
+          {
+            "script": "",
+            "value": 1000
+          }
+        ]
+      },
+      {
+        "exception": "Transaction is missing signatures",
+        "hex": "",
+        "inputs": [
+          {
+            "hash": "",
+            "index": 0
+          },
+          {
+            "hash": "",
+            "index": 1
+          }
+        ],
+        "outputs": [
+          {
+            "script": "",
+            "value": 1000
+          }
+        ],
+        "signatures": [
+          {
+            "wif": "",
+            "index": 0
+          }
+        ]
+      }
+    ],
+    "fromTransaction": [
+      {
+        "exception": "Transaction contains unsupported script types",
+        "hex": ""
+      }
+    ]
+  }
+}
diff --git a/test/transaction_builder.js b/test/transaction_builder.js
new file mode 100644
index 0000000..4a7f0ea
--- /dev/null
+++ b/test/transaction_builder.js
@@ -0,0 +1,228 @@
+var assert = require('assert')
+var ecdsa = require('../src/ecdsa')
+var scripts = require('../src/scripts')
+
+var Address = require('../src/address')
+var BigInteger = require('bigi')
+var ECKey = require('../src/eckey')
+var Script = require('../src/script')
+var Transaction = require('../src/transaction')
+var TransactionBuilder = require('../src/transaction_builder')
+
+var fixtures = require('./fixtures/transaction_builder')
+
+describe('TransactionBuilder', function() {
+  var privAddress, privScript
+  var prevTx, prevTxHash
+  var privKey
+  var txb
+
+  beforeEach(function() {
+    txb = new TransactionBuilder()
+
+    prevTx = new Transaction()
+    prevTx.addOutput('1BgGZ9tcN4rm9KBzDn7KprQz87SZ26SAMH', 0)
+    prevTx.addOutput('1cMh228HTCiwS8ZsaakH8A8wze1JR5ZsP', 1)
+    prevTxHash = prevTx.getHash()
+    prevTxId = prevTx.getId()
+
+    privKey = new ECKey(BigInteger.ONE, false)
+    privAddress = privKey.pub.getAddress()
+    privScript = privAddress.toOutputScript()
+    value = 10000
+  })
+
+  describe('addInput', function() {
+    it('accepts a txHash and index', function() {
+      var vin = txb.addInput(prevTxHash, 1)
+      assert.equal(vin, 0)
+
+      var txin = txb.tx.ins[0]
+      assert.equal(txin.hash, prevTxHash)
+      assert.equal(txin.index, 1)
+      assert.equal(txb.prevOutScripts[0], undefined)
+    })
+
+    it('accepts a txHash, index and scriptPubKey', function() {
+      var vin = txb.addInput(prevTxHash, 1, prevTx.outs[1].script)
+      assert.equal(vin, 0)
+
+      var txin = txb.tx.ins[0]
+      assert.equal(txin.hash, prevTxHash)
+      assert.equal(txin.index, 1)
+      assert.equal(txb.prevOutScripts[0], prevTx.outs[1].script)
+    })
+
+    it('accepts a prevTx and index', function() {
+      var vin = txb.addInput(prevTx, 1)
+      assert.equal(vin, 0)
+
+      var txin = txb.tx.ins[0]
+      assert.deepEqual(txin.hash, prevTxHash)
+      assert.equal(txin.index, 1)
+      assert.equal(txb.prevOutScripts[0], prevTx.outs[1].script)
+    })
+
+    it('returns the input index', function() {
+      assert.equal(txb.addInput(prevTxHash, 0), 0)
+      assert.equal(txb.addInput(prevTxHash, 1), 1)
+    })
+
+    it('throws if a Tx and prevOutScript is given', function() {
+      assert.throws(function() {
+        txb.addInput(prevTx, 0, privScript)
+      }, /Unnecessary Script provided/)
+    })
+
+    it('throws if prevOutScript is not supported', function() {
+      assert.throws(function() {
+        txb.addInput(prevTxHash, 0, Script.EMPTY)
+      }, /PrevOutScript not supported \(nonstandard\)/)
+    })
+
+    it('throws if SIGHASH_ALL has been used to sign any existing scriptSigs', function() {
+      txb.addInput(prevTxHash, 0)
+      txb.sign(0, privKey)
+
+      assert.throws(function() {
+        txb.addInput(prevTxHash, 0)
+      }, /No, this would invalidate signatures/)
+    })
+  })
+
+  describe('addOutput', function() {
+    it('throws if SIGHASH_ALL has been used to sign any existing scriptSigs', function() {
+      txb.addInput(prevTxHash, 0)
+      txb.addOutput(privScript, value)
+      txb.sign(0, privKey)
+
+      assert.throws(function() {
+        txb.addOutput(privScript, 9000)
+      }, /No, this would invalidate signatures/)
+    })
+  })
+
+  describe('sign', function() {
+    describe('when prevOutScript is undefined', function() {
+      it('assumes pubKeyHash', function() {
+        txb.addInput(prevTxHash, 0)
+        txb.sign(0, privKey)
+
+        assert.strictEqual(txb.signatures[0].redeemScript, undefined)
+        assert.equal(txb.signatures[0].scriptType, 'pubkeyhash')
+      })
+    })
+
+    describe('when redeemScript is defined', function() {
+      it('assumes scriptHash', function() {
+        txb.addInput(prevTxHash, 0)
+        txb.sign(0, privKey, privScript)
+
+        assert.equal(txb.signatures[0].redeemScript, privScript)
+      })
+
+      it('throws if prevOutScript is not P2SH', function() {
+        txb.addInput(prevTx, 0)
+
+        assert.throws(function() {
+          txb.sign(0, privKey, privScript)
+        }, /PrevOutScript must be P2SH/)
+      })
+
+      it('throws if redeemScript is P2SH', function() {
+        txb.addInput(prevTxHash, 0)
+
+        var privScriptP2SH = scripts.scriptHashOutput(privScript.getHash())
+
+        assert.throws(function() {
+          txb.sign(0, privKey, privScriptP2SH)
+        }, /RedeemScript can\'t be P2SH/)
+      })
+
+      it('throws if redeemScript not supported', function() {
+        txb.addInput(prevTxHash, 0)
+
+        assert.throws(function() {
+          txb.sign(0, privKey, Script.EMPTY)
+        }, /RedeemScript not supported \(nonstandard\)/)
+      })
+    })
+  })
+
+  describe('build', function() {
+    fixtures.valid.build.forEach(function(f) {
+      it('builds the correct transaction', function() {
+        f.inputs.forEach(function(input) {
+          var prevTx
+          if (input.prevTx.length === 64) {
+            prevTx = input.prevTx
+          } else {
+            prevTx = Transaction.fromHex(input.prevTx)
+          }
+
+          txb.addInput(prevTx, input.index)
+        })
+
+        f.outputs.forEach(function(output) {
+          var script = Script.fromASM(output.script)
+
+          txb.addOutput(script, output.value)
+        })
+
+        f.inputs.forEach(function(input, index) {
+          var redeemScript
+
+          if (input.redeemScript) {
+            redeemScript = Script.fromASM(input.redeemScript)
+          }
+
+          input.privKeys.forEach(function(wif) {
+            var privKey = ECKey.fromWIF(wif)
+
+            txb.sign(index, privKey, redeemScript)
+          })
+        })
+
+        var tx = txb.build()
+
+        assert.equal(tx.getId(), f.txid)
+      })
+    })
+
+    it('throws if Transaction has no inputs', function() {
+      txb.addOutput(privScript, value)
+
+      assert.throws(function() {
+        txb.build()
+      }, /Transaction has no inputs/)
+    })
+
+    it('throws if Transaction has no outputs', function() {
+      txb.addInput(prevTxHash, 0)
+
+      assert.throws(function() {
+        txb.build()
+      }, /Transaction has no outputs/)
+    })
+
+    it('throws if Transaction has no signatures', function() {
+      txb.addInput(prevTxHash, 0)
+      txb.addOutput(privScript, value)
+
+      assert.throws(function() {
+        txb.build()
+      }, /Transaction is missing signatures/)
+    })
+
+    it('throws if Transaction has not enough signatures', function() {
+      txb.addInput(prevTxHash, 0)
+      txb.addInput(prevTxHash, 1)
+      txb.addOutput(privScript, value)
+      txb.sign(0, privKey)
+
+      assert.throws(function() {
+        txb.build()
+      }, /Transaction is missing signatures/)
+    })
+  })
+})