diff --git a/lib/chai/core/assertions.js b/lib/chai/core/assertions.js
index 4da99927..0a55c58e 100644
--- a/lib/chai/core/assertions.js
+++ b/lib/chai/core/assertions.js
@@ -239,6 +239,13 @@ Assertion.addProperty('all', function () {
   flag(this, 'any', false);
 });
 
+const functionTypes = {
+  'function': ['function', 'asyncfunction', 'generatorfunction', 'asyncgeneratorfunction'],
+  'asyncfunction': ['asyncfunction', 'asyncgeneratorfunction'],
+  'generatorfunction': ['generatorfunction', 'asyncgeneratorfunction'],
+  'asyncgeneratorfunction': ['asyncgeneratorfunction']
+}
+
 /**
  * ### .a(type[, msg])
  *
@@ -298,18 +305,27 @@ Assertion.addProperty('all', function () {
  * @namespace BDD
  * @api public
  */
-
 function an (type, msg) {
   if (msg) flag(this, 'message', msg);
   type = type.toLowerCase();
   var obj = flag(this, 'object')
     , article = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(type.charAt(0)) ? 'an ' : 'a ';
 
-  this.assert(
-      type === _.type(obj).toLowerCase()
-    , 'expected #{this} to be ' + article + type
-    , 'expected #{this} not to be ' + article + type
-  );
+  const detectedType = _.type(obj).toLowerCase();
+
+  if (functionTypes['function'].includes(type)) {
+    this.assert(
+        functionTypes[type].includes(detectedType)
+      , 'expected #{this} to be ' + article + type
+      , 'expected #{this} not to be ' + article + type
+    );
+  } else {
+    this.assert(
+      type === detectedType
+      , 'expected #{this} to be ' + article + type
+      , 'expected #{this} not to be ' + article + type
+    );
+  }
 }
 
 Assertion.addChainableMethod('an', an);
@@ -672,6 +688,43 @@ Assertion.addProperty('true', function () {
   );
 });
 
+/**
+ * ### .callable
+ *
+ * Asserts that the target a callable function.
+ *
+ *     expect(console.log).to.be.callable;
+ *
+ * A custom error message can be given as the second argument to `expect`.
+ *
+ *     expect('not a function', 'nooo why fail??').to.be.callable;
+ *
+ * @name callable
+ * @namespace BDD
+ * @api public
+ */
+Assertion.addProperty('callable', function () {
+  const val = flag(this, 'object')
+  const ssfi = flag(this, 'ssfi')
+  const message = flag(this, 'message')
+  const msg = message ? `${message}: ` : ''
+  const negate = flag(this, 'negate');
+
+  const assertionMessage = negate ?
+    `${msg}expected ${_.inspect(val)} not to be a callable function` : 
+    `${msg}expected ${_.inspect(val)} to be a callable function`;
+
+  const isCallable = ['Function', 'AsyncFunction', 'GeneratorFunction', 'AsyncGeneratorFunction'].includes(_.type(val));
+
+  if ((isCallable && negate) || (!isCallable && !negate)) {
+    throw new AssertionError(
+      assertionMessage,
+      undefined,
+      ssfi
+    );
+  }
+});
+
 /**
  * ### .false
  *
diff --git a/lib/chai/interface/assert.js b/lib/chai/interface/assert.js
index 77af8ead..24ceff36 100644
--- a/lib/chai/interface/assert.js
+++ b/lib/chai/interface/assert.js
@@ -8,6 +8,7 @@ import * as chai from '../../../index.js';
 import {Assertion} from '../assertion.js';
 import {flag, inspect} from '../utils/index.js';
 import {AssertionError} from 'assertion-error';
+import {type} from '../utils/type-detect.js';
 
 /**
  * ### assert(expression, message)
@@ -553,41 +554,39 @@ assert.isDefined = function (val, msg) {
 };
 
 /**
- * ### .isFunction(value, [message])
+ * ### .isCallable(value, [message])
  *
- * Asserts that `value` is a function.
+ * Asserts that `value` is a callable function.
  *
  *     function serveTea() { return 'cup of tea'; };
- *     assert.isFunction(serveTea, 'great, we can have tea now');
+ *     assert.isCallable(serveTea, 'great, we can have tea now');
  *
- * @name isFunction
+ * @name isCallable
  * @param {Mixed} value
  * @param {String} message
  * @namespace Assert
  * @api public
  */
-
-assert.isFunction = function (val, msg) {
-  new Assertion(val, msg, assert.isFunction, true).to.be.a('function');
-};
+assert.isCallable = function (val, msg) {
+  new Assertion(val, msg, assert.isCallable, true).is.callable;
+}
 
 /**
- * ### .isNotFunction(value, [message])
+ * ### .isNotCallable(value, [message])
  *
- * Asserts that `value` is _not_ a function.
+ * Asserts that `value` is _not_ a callable function.
  *
  *     var serveTea = [ 'heat', 'pour', 'sip' ];
- *     assert.isNotFunction(serveTea, 'great, we have listed the steps');
+ *     assert.isNotCallable(serveTea, 'great, we have listed the steps');
  *
- * @name isNotFunction
+ * @name isNotCallable
  * @param {Mixed} value
  * @param {String} message
  * @namespace Assert
  * @api public
  */
-
-assert.isNotFunction = function (val, msg) {
-  new Assertion(val, msg, assert.isNotFunction, true).to.not.be.a('function');
+assert.isNotCallable = function (val, msg) {
+  new Assertion(val, msg, assert.isNotCallable, true).is.not.callable;
 };
 
 /**
@@ -3104,4 +3103,6 @@ assert.isNotEmpty = function(val, msg) {
 ('isFrozen', 'frozen')
 ('isNotFrozen', 'notFrozen')
 ('isEmpty', 'empty')
-('isNotEmpty', 'notEmpty');
+('isNotEmpty', 'notEmpty')
+('isCallable', 'isFunction')
+('isNotCallable', 'isNotFunction')
diff --git a/test/assert.js b/test/assert.js
index 8462fd5e..22373a35 100644
--- a/test/assert.js
+++ b/test/assert.js
@@ -139,6 +139,20 @@ describe('assert', function () {
     assert.typeOf('test', 'string');
     assert.typeOf(true, 'boolean');
     assert.typeOf(5, 'number');
+    
+    assert.typeOf(() => {}, 'function');
+    assert.typeOf(function() {}, 'function');
+    assert.typeOf(async function() {}, 'asyncfunction');
+    assert.typeOf(function*() {}, 'generatorfunction');
+    assert.typeOf(async function*() {}, 'asyncgeneratorfunction');
+    
+    err(function () {
+      assert.typeOf(5, 'function', 'blah');
+    }, "blah: expected 5 to be a function");
+    
+    err(function () {
+      assert.typeOf(function() {}, 'asyncfunction', 'blah');
+    }, "blah: expected [Function] to be an asyncfunction");
 
     if (typeof Symbol === 'function') {
       assert.typeOf(Symbol(), 'symbol');
@@ -151,10 +165,20 @@ describe('assert', function () {
 
   it('notTypeOf', function () {
     assert.notTypeOf('test', 'number');
+    
+    assert.notTypeOf(() => {}, 'string');
+    assert.notTypeOf(function() {}, 'string');
+    assert.notTypeOf(async function() {}, 'string');
+    assert.notTypeOf(function*() {}, 'string');
+    assert.notTypeOf(async function*() {}, 'string');
 
     err(function () {
       assert.notTypeOf(5, 'number', 'blah');
     }, "blah: expected 5 not to be a number");
+    
+    err(function () {
+      assert.notTypeOf(() => {}, 'function', 'blah');
+    }, "blah: expected [Function] not to be a function");
   });
 
   it('instanceOf', function() {
@@ -521,13 +545,41 @@ describe('assert', function () {
     }, "blah: expected undefined to not equal undefined");
   });
 
+  it('isCallable', function() {
+    var func = function() {};
+    assert.isCallable(func);
+
+    var func = async function() {};
+    assert.isCallable(func);
+
+    var func = function* () {}
+    assert.isCallable(func);
+
+    var func = async function* () {}
+    assert.isCallable(func);
+
+    err(function () {
+      assert.isCallable({}, 'blah');
+    }, "blah: expected {} to be a callable function");
+  });
+  
+  it('isNotCallable', function() {
+    assert.isNotCallable(false);
+    assert.isNotCallable(10);
+    assert.isNotCallable('string');
+
+    err(function () {
+      assert.isNotCallable(function() {}, 'blah');
+    }, "blah: expected [Function] not to be a callable function");
+  });
+
   it('isFunction', function() {
     var func = function() {};
     assert.isFunction(func);
 
     err(function () {
       assert.isFunction({}, 'blah');
-    }, "blah: expected {} to be a function");
+    }, "blah: expected {} to be a callable function");
   });
 
   it('isNotFunction', function () {
@@ -535,7 +587,7 @@ describe('assert', function () {
 
     err(function () {
       assert.isNotFunction(function () {}, 'blah');
-    }, "blah: expected [Function] not to be a function");
+    }, "blah: expected [Function] not to be a callable function");
   });
 
   it('isArray', function() {
diff --git a/test/expect.js b/test/expect.js
index 2dd08ef2..d5247655 100644
--- a/test/expect.js
+++ b/test/expect.js
@@ -385,6 +385,90 @@ describe('expect', function () {
     }, "blah: expected 5 not to be a number");
   });
 
+  it('callable', function() {
+    expect(function() {}).to.be.callable;
+    expect(async function() {}).to.be.callable;
+    expect(function*() {}).to.be.callable;
+    expect(async function*() {}).to.be.callable;
+    
+    expect('foobar').to.not.be.callable;
+    
+    err(function(){
+      expect('foobar', 'blah').to.be.callable;
+    }, "blah: expected 'foobar' to be a callable function");
+    
+    err(function(){
+      expect(function() {}, 'blah').to.not.be.callable;
+    }, "blah: expected [Function] not to be a callable function");
+  });
+    
+  it('function', function() {
+    expect(function() {}).to.be.a('function');
+    expect(async function() {}).to.be.a('function');
+    expect(function*() {}).to.be.a('function');
+    expect(async function*() {}).to.be.a('function');
+    
+    expect('foobar').to.not.be.a('function');
+    
+    err(function(){
+      expect('foobar').to.be.a('function', 'blah');
+    }, "blah: expected 'foobar' to be a function");
+
+    err(function(){
+      expect(function() {}).to.not.be.a('function', 'blah');
+    }, "blah: expected [Function] not to be a function");
+    
+    err(function(){
+      expect(function() {}, 'blah').to.not.be.a('function');
+    }, "blah: expected [Function] not to be a function");
+  })
+
+  it('asyncFunction', function() {
+    expect(async function() {}).to.be.a('AsyncFunction');
+    expect(async function*() {}).to.be.a('AsyncFunction');
+
+    err(function(){
+      expect('foobar').to.be.a('asyncfunction', 'blah');
+    }, "blah: expected 'foobar' to be an asyncfunction");
+
+    err(function(){
+      expect(async function() {}).to.not.be.a('asyncfunction', 'blah');
+    }, "blah: expected [AsyncFunction] not to be an asyncfunction");
+    
+    err(function(){
+      expect(async function() {}, 'blah').to.not.be.a('asyncfunction');
+    }, "blah: expected [AsyncFunction] not to be an asyncfunction");
+  })
+  
+  it('generatorFunction', function() {
+    expect(function*() {}).to.be.a('generatorFunction');
+    expect(async function*() {}).to.be.a('generatorFunction');
+    
+    err(function(){
+      expect('foobar').to.be.a('generatorfunction', 'blah');
+    }, "blah: expected 'foobar' to be a generatorfunction");
+    
+    err(function(){
+      expect(function*() {}).to.not.be.a('generatorfunction', 'blah');
+    }, "blah: expected [GeneratorFunction] not to be a generatorfunction");
+    
+    err(function(){
+      expect(function*() {}, 'blah').to.not.be.a('generatorfunction');
+    }, "blah: expected [GeneratorFunction] not to be a generatorfunction");
+  })
+
+  it('asyncGeneratorFunction', function() {
+    expect(async function*() {}).to.be.a('asyncGeneratorFunction');
+    
+    err(function(){
+      expect(async function() {}, 'blah').to.be.a('asyncgeneratorfunction');
+    }, "blah: expected [AsyncFunction] to be an asyncgeneratorfunction");
+    
+    err(function(){
+      expect(async function*() {}, 'blah').to.not.be.a('asyncgeneratorfunction');
+    }, "blah: expected [AsyncGeneratorFunction] not to be an asyncgeneratorfunction");
+  })
+
   it('instanceof', function(){
     function Foo(){}
     expect(new Foo()).to.be.an.instanceof(Foo);
diff --git a/test/should.js b/test/should.js
index 0da30610..adb94d94 100644
--- a/test/should.js
+++ b/test/should.js
@@ -337,6 +337,18 @@ describe('should', function() {
     }, "expected '' to be false")
   });
 
+  it("callable", function () {
+    (function () {}).should.be.callable;
+    (async function () {}).should.be.callable;
+    (function* () {}).should.be.callable;
+    (async function* () {}).should.be.callable;
+    true.should.not.be.callable;
+
+    err(function () {
+      "".should.be.callable;
+    }, "expected '' to be a callable function");
+  });
+
   it('null', function(){
     (0).should.not.be.null;