Why do international institutions vary so widely in terms of such key institutional features as membership, scope, and flexibility? We argue that international actors are goal-seeking agents who make specific institutional design choices to solve the particular cooperation problems they face in different issue-areas. In this article we introduce the theoretical framework of the Rational Design project. We identify five important features of institutions—membership, scope, centralization, control, and flexibility—and explain their variation in terms of four independent variables that characterize different cooperation problems: distribution, number of actors, enforcement, and uncertainty. We draw on rational choice theory to develop a series of empirically falsifiable conjectures that explain this institutional variation. The authors of the articles in this special issue ofInternational Organizationevaluate the conjectures in specific issue-areas and the overall Rational Design approach.