You're off to a pretty good start. By now, you should be starting to feel comfortable with the basic concepts behind Test-Driven Development. You know the basic premise behind TDD and how to write a unit test in C# and JavaScript.

In this chapter:

• We'll cover more of the practices behind TDD
• Specific advice will be given on how to avoid pitfalls along the way
• We'll explain the importance of defining and testing boundaries, abstracting away third-party code (including the .NET Framework)
• We'll begin to introduce more advanced concepts, such as spies, mocks, and fakes

First, let's cover some issues you may run into while trying to test an existing application. Hopefully, this will help you avoid problems in your next green-field application.