Regression testing, which attempts to validate modified software and ensure that no new errors are introduced into previously tested code, is used extensively during maintenance of evolving software. Despite efforts to reduce its cost, regression testing remains one of the most expensive activities performed during a software system's lifetime. Because regression testing is important and expensive, many researchers have focused on ways to make it more efficient and effective. Research on regression testing spans a wide variety of topics, including test environments and automation, capture-playback mechanisms, regression-test selection, coverage identification, test suite maintenance, regression testability, and regression-testing process. This paper discusses the state of the art in several important aspects of regression testing, and presents some promising areas for future research.