Writing unit tests is not easy. Most of the time, we need to test code that uses complex collaborators such as databases, application servers, or software modules that have not been written yet. We also may need to deal with conditions that are difficult to generate in a test environment. Setting up these dependencies may require a considerable amount of time and energy, counteracting the benefits of having automated tests. This article looks at Mock Objects, a testing technique from the XP community that offers a way to test our code in isolation by simulating those external dependencies. As with any other tool, we need to be careful and avoid overusing them.