Analysts of international politics can measure and explain the effect of international environmental institutions on the behavior of states and other actors and on the natural environment in three steps. First, we measure the outcome to be explained in terms of goal attainment, defined as the difference, over time or across cases, between actor behavior or the state of the natural environment on dimensions identified by institutional goals and certain end points determined by institutional goals. Second, we assess the effect of an institution in terms of the extent to which the existence or operation of the institution contributes, ceteris paribus, to variation in goal attainment. We transform these two variables into a score of institutional effectiveness to indicate the degree to which institutions contribute to the resolution of the environmental problems that motivate their establishment. Third, we analyze the relationship between institutional effectiveness and specific dimensions of institutional design—such as decision-making rules, membership and access conditions, and the compliance system.