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 of International Organization evaluate the conjectures in specific issue-areas and the overall Rational Design approach.