The effectivness of international environmental regimes have been studied for two decades now and considerable advancements have been made both in terms of how to measure performance and how to explain performance. There is agreement on how to measure effectiveness but a wide variety of perspectives exist regarding how it shall be explained. But there is agreement that both regime attributes as well as non-institutional factors matter. Still, it is argued here that it is exceedingly difficult to decide exactly how effective regime are, exemplified by a brief analaysis of the climate regime. Empirical studies on effectiveness peaked in the 1990s but little has been done in this millenium. More update empirical reserach is therefore needed, but the scope needs to be wider than the traditional problem solving effectiveness persective.