 <p>Is it possible that we are in a panic and are getting confused between unit tests and performance tests? Is it that your application works fine with few users, but starts throwing errors when under heavier load? If so, unit tests are not the answer. Unit tests != Load tests.</p> <p>If unit tests are in fact the answer, retrofitting unit tests is a good idea as it will help clean up the code. Just be prepared to refactor a lot. Code written with TDD turns out looking a lot different than code written without TDD. In my case, I had a method HandleDisposition() which took care of a lot of cases. This kind of method would not have existed if we had written the code with TDD. When retrofitting unit tests, we refactored that function and now have methods like XDisposition(), YDisposition(), ZDisposition(), which are a lot easier to write unit tests against.</p>