Mock objects are simulated objects that mimic the behavior of real objects in controlled ways. Because many code modules interact with external entities-things like databases, networks, file systems, third-party frameworks, and even the clock-these entities often cause us big-time trouble during unit testing. These entities can slow down our unit tests, produce unpredictable results, and have dangerous side effects. The best unit tests are decoupled from these external entities. Rather than try to control the entities, you can create mock objects to simulate their functionality. With a tangible example in the form of a short play, Rob Myers introduces mock objects and provides a brief history of their "relatives"-stubs and fakes. Then, with an animated, nearly-worst-case example, Rob presents code he developed to "mock out" nasty dependencies and create safe, predictable unit tests.