Writing good unit tests is made much easier by **dependency injection**. This lets you separate your code's behavior from that of your dependencies. Many people use **mocks** to add dependencies to unit tests. I think this is usually a mistake.

Always interesting to hear different points of view on this subject. Personally I think mocks make sense to capture complex sets of interactions or otherwise difficult to reach error conditions, so I don't think it's a do or do-not kind of thing.