In response to a much nicer blog entry, that can be found here . There are actually several distinct "tests" that make up usual unit tests, among them two that really do stand out: one kind of testing to test method flows, one to test some sort of computation. Mock objects are for the purpose of testing method flows. A method flow is a series of message transmissions to dependent objects. The control flow logic inside the method(the ifs and whiles) will alter the flow in repsonse to the parameters of the method call parameters passed by calling the method under test, depending on the state of the object that contains the method under test and the return values of the external method calls(aka responses to the messages sent). There should be one test method for every branch of an if statement, and usuale some sort of mock control objects in the mock framework will handle loop checking. BTW: I partly use message transmission instead of method invocation to include other kind...