Occasionally, I find that TestDriven.NET's "Test In Debugger" feature quits working. TD.NET creates a surrogate project for the Test In Debugger feature. The project may or may not show up in your solution, but it will be stored in the same directory as your unit test project. So, for example, if you have a project Foo.Tests, you'll find another project in the same directory Foo.Tests.Surrogate.csproj (or whatever extension is appropriate). When I get to the stage that "Test in Debugger" quits working, I've found I have to delete the surrogate project to get it working again. I have no idea why this happens, but I've been frustrated by it enough that I figured that somebody else must have seen this happen as well.