This article provides a process to integrate Test-Driven Development in the maintenance of a .NET application. You will first write the tests that will prove your new code gets the same results as the legacy code. Then you isolate new code into separate methods or classes that you can test independently. You break down the requirements into simple valid tests. Finally you check to see that you have not introduced a new bug. While you are doing this, let Visual Studio generate the utility code for you and find a tool to run your tests continuously like AutoTest.NET.