diff --git a/index.mjs b/index.mjs
index 822968b..d0a1ebb 100644
--- a/index.mjs
+++ b/index.mjs
@@ -1,7 +1,10 @@
 import Eltro from './lib/eltro.mjs'
 import assert from './lib/assert.mjs'
+import { spy, stub } from './lib/sinon.mjs'
 
 export {
     Eltro,
     assert,
+    spy,
+    stub,
 }
diff --git a/lib/sinon.mjs b/lib/sinon.mjs
new file mode 100644
index 0000000..2feaaa4
--- /dev/null
+++ b/lib/sinon.mjs
@@ -0,0 +1,55 @@
+const indexMap = [
+  'firstCall',
+  'secondCall',
+  'thirdCall',
+]
+
+export function spy() {
+  return stub()
+}
+
+export function stub(returnFunc = null) {
+  if (returnFunc && typeof(returnFunc) !== 'function') {
+    throw new Error('stub() was called with non-function argument')
+  }
+  let returner = returnFunc ? returnFunc : null
+  let calls = []
+  let func = function(...args) {
+    func.called = true
+    calls.push(args)
+    if (func.callCount < indexMap.length) {
+      func[indexMap[func.callCount]] = args
+    }
+    func.callCount++
+    if (returner) {
+      return returner(...args)
+    }
+  }
+  func.called = false
+  func.callCount = 0
+  func.onCall = function(i) {
+    return calls[i]
+  }
+  func.returns = function(data) {
+    returner = function() { return data }
+  }
+  func.returnWith = function(returnFunc) {
+    if (typeof(returnFunc) !== 'function') {
+      throw new Error('stub() was called with non-function argument')
+    }
+    returner = returnFunc
+  }
+  func.throws = function(data) {
+    returner = function() { throw data }
+  }
+  func.resolves = function(data) {
+    returner = function() { return Promise.resolve(data) }
+  }
+  func.rejects = function(data) {
+    returner = function() { return Promise.reject(data) }
+  }
+  for (let i = 0; i < indexMap.length; i++) {
+    func[indexMap[i]] = null
+  }
+  return func
+}
diff --git a/test/sinon.test.mjs b/test/sinon.test.mjs
new file mode 100644
index 0000000..c80ef96
--- /dev/null
+++ b/test/sinon.test.mjs
@@ -0,0 +1,182 @@
+import assert from '../lib/assert.mjs'
+import t from '../lib/eltro.mjs'
+import { spy, stub } from '../index.mjs'
+
+[spy, stub].forEach(function(tester, i) {
+  t.describe(`#${i === 0 ? 'spy' : 'stub'}()`, function() {
+    t.test('should keep track of call count', function() {
+      let spyer = tester()
+      assert.strictEqual(spyer.callCount, 0)
+      assert.notOk(spyer.called)
+      assert.strictEqual(spyer.firstCall, null)
+      assert.strictEqual(spyer.secondCall, null)
+      assert.strictEqual(spyer.thirdCall, null)
+  
+      spyer()
+      assert.strictEqual(spyer.callCount, 1)
+      assert.ok(spyer.called)
+      assert.deepStrictEqual(spyer.firstCall, [])
+      assert.strictEqual(spyer.secondCall, null)
+      assert.strictEqual(spyer.thirdCall, null)
+  
+      spyer()
+      assert.strictEqual(spyer.callCount, 2)
+      assert.ok(spyer.called)
+      assert.deepStrictEqual(spyer.firstCall, [])
+      assert.deepStrictEqual(spyer.secondCall, [])
+      assert.strictEqual(spyer.thirdCall, null)
+  
+      spyer()
+      assert.strictEqual(spyer.callCount, 3)
+      assert.ok(spyer.called)
+      assert.deepStrictEqual(spyer.firstCall, [])
+      assert.deepStrictEqual(spyer.secondCall, [])
+      assert.deepStrictEqual(spyer.thirdCall, [])
+      
+      spyer()
+      assert.strictEqual(spyer.callCount, 4)
+    })
+  
+    t.test('should keep track of arguments used to call', function() {
+      const assertFirstArgs = 'asdf'
+      const assertSecondArgs = { a: 1 }
+      const assertThirdArgs = [{ b: 1 }, { c: 2 }]
+      let spyer = tester()
+      assert.notOk(spyer.called)
+      
+      spyer(assertFirstArgs)
+      spyer(assertSecondArgs)
+      spyer(assertThirdArgs[0], assertThirdArgs[1])
+      assert.strictEqual(spyer.callCount, 3)
+  
+      assert.strictEqual(spyer.onCall(0)[0], assertFirstArgs)
+      assert.strictEqual(spyer.onCall(1)[0], assertSecondArgs)
+      assert.strictEqual(spyer.onCall(2)[0], assertThirdArgs[0])
+      assert.strictEqual(spyer.onCall(2)[1], assertThirdArgs[1])
+  
+      assert.strictEqual(spyer.firstCall[0], assertFirstArgs)
+      assert.strictEqual(spyer.secondCall[0], assertSecondArgs)
+      assert.strictEqual(spyer.thirdCall[0], assertThirdArgs[0])
+      assert.strictEqual(spyer.thirdCall[1], assertThirdArgs[1])
+    })
+  })
+})
+
+t.describe('#stub()', function() {
+  t.test('should support returns', function() {
+    const assertInput = { a : 1 }
+    const assertReturns = { b: 3 }
+    let s = stub()
+    assert.strictEqual(s(), undefined)
+    s.returns(assertReturns)
+    assert.strictEqual(s(assertInput), assertReturns)
+    assert.ok(s.called)
+    assert.strictEqual(s.callCount, 2)
+    assert.strictEqual(s.secondCall[0], assertInput)
+  })
+  
+  t.test('should support throws', function() {
+    const assertInput = { a : 1 }
+    const assertThrows = new Error('testety test')
+    let s = stub()
+    assert.strictEqual(s(), undefined)
+    s.throws(assertThrows)
+    assert.throws(function() {
+      s(assertInput)
+    }, assertThrows)
+    assert.ok(s.called)
+    assert.strictEqual(s.callCount, 2)
+    assert.strictEqual(s.secondCall[0], assertInput)
+  })
+
+  t.test('should support resolves', async function() {
+    const assertInput = { a : 1 }
+    const assertReturns = { b: 3 }
+    let s = stub()
+    assert.strictEqual(s(), undefined)
+    s.resolves(assertReturns)
+    let promiser = s(assertInput)
+    assert.ok(promiser.then)
+    assert.strictEqual(typeof(promiser.then), 'function')
+    let output = await promiser
+    assert.strictEqual(output, assertReturns)
+  })
+  
+  t.test('should support rejects', async function() {
+    const assertInput = { a : 1 }
+    const assertReturns = { b: 3 }
+    let s = stub()
+    assert.strictEqual(s(), undefined)
+    s.rejects(assertReturns)
+    let promiser = s(assertInput)
+    assert.ok(promiser.then)
+    assert.strictEqual(typeof(promiser.then), 'function')
+    let output = await assert.isRejected(promiser)
+    assert.strictEqual(output, assertReturns)
+  })
+  
+  t.test('should support custom function overwrite', async function() {
+    const assertInput = { a : 1 }
+    const assertReturns = { b: 3 }
+    let ourCallCount = 50
+    let s = stub(function(input) {
+      ourCallCount++
+      assert.strictEqual(input, assertInput)
+      return assertReturns
+    })
+    assert.strictEqual(s(assertInput), assertReturns)
+    assert.ok(s.called)
+    assert.strictEqual(s.callCount, 1)
+    assert.strictEqual(ourCallCount, 51)
+    assert.strictEqual(s.firstCall[0], assertInput)
+    
+    assert.throws(function() {
+      s(null)
+    })
+  })
+  
+  t.test('should support custom function overwrite after the fact', async function() {
+    const assertInput = { a : 1 }
+    const assertReturns = { b: 3 }
+    let ourCallCount = 0
+    let s = stub()
+    assert.strictEqual(s(null), undefined)
+    s.returnWith(function(input) {
+      ourCallCount++
+      assert.strictEqual(input, assertInput)
+      return assertReturns
+    })
+    assert.strictEqual(s(assertInput), assertReturns)
+    assert.ok(s.called)
+    assert.strictEqual(s.callCount, 2)
+    assert.strictEqual(ourCallCount, 1)
+    assert.strictEqual(s.secondCall[0], assertInput)
+    
+    assert.throws(function() {
+      s(null)
+    })
+  })
+
+  t.test('should throw if parameter is something else than a function', function() {
+    assert.ok(stub(null))
+    assert.ok(stub(0))
+    assert.ok(stub(''))
+    assert.strictEqual(stub(null)(), undefined)
+    assert.strictEqual(stub(0)(), undefined)
+    assert.strictEqual(stub('')(), undefined)
+    assert.throws(function() { stub([]) })
+    assert.throws(function() { stub({}) })
+    assert.throws(function() { stub(123) })
+    assert.throws(function() { stub('asdf') })
+  })
+
+  t.test('should throw if returnWith is called with something else than a function', function() {
+    assert.throws(function() { stub().returnWith(0) })
+    assert.throws(function() { stub().returnWith('') })
+    assert.throws(function() { stub().returnWith(null) })
+    assert.throws(function() { stub().returnWith([]) })
+    assert.throws(function() { stub().returnWith({}) })
+    assert.throws(function() { stub().returnWith(123) })
+    assert.throws(function() { stub().returnWith('asdf') })
+  })
+})