Test suites, once created, rarely remain static. Just like the application they are testing, they evolve throughout their lifetime. Test obsolescence is probably the most known reason for test-suite evolution - test cases cease to work because of changes in the code and must be suitably repaired. Repairing existing test cases manually, however, can be extremely time consuming, especially for large test suites, which has motivated the recent development of automated test-repair techniques. We believe that, for developing effective repair techniques that are applicable in real-world scenarios, a fundamental prerequisite is a thorough understanding of how test cases evolve in practice. Without such knowledge, we risk to develop techniques that may work well for only a small number of tests or, worse, that may not work at all in most realistic cases. Unfortunately, to date there are no studies in the literature that investigate how test suites evolve. To tackle this problem, in this paper we present a technique for studying test-suite evolution, a tool that implements the technique, and an extensive empirical study in which we used our technique to study many versions of six real-world programs and their unit test suites. This is the first study of this kind, and our results reveal several interesting aspects of test-suite evolution. In particular, our findings show that test repair is just one possible reason for test-suite evolution, whereas most changes involve refactorings, deletions, and additions of test cases. Our results also show that test modifications tend to involve complex, and hard-to-automate, changes to test cases, and that existing test-repair techniques that focus exclusively on assertions may have limited practical applicability. More generally, our findings provide initial insight on how test cases are added, removed, and modified in practice, and can guide future research efforts in the area of test-suite evolution. © 2012 ACM.