Testability is the quality that refers to how easily a software class, module, or system can be tested. Automated unit testing is the fundamental quality activity of most agile software development methods. Unfortunately, it is difficult to build sophisticated new systems that properly support testability. And many parts of legacy systems are virtually impossible to effectively unit-test. The core reason for this situation is that many common design practices -- for example, static classes and methods, global state, and poor object isolation -- are contrary to the goal of testability. Fortunately, we have the means to make our code more testable, and the automated test technology to _execute