International institutions vary widely in terms of key institutional features such as membership, scope, and flexibility. Barbara Koremenos, Charles Lipson, and Duncan Snidal argue that this is so because international actors are goal-seeking agents who make specific institutional design choices to solve the particular cooperation problems they face in different issue-areas. Using a Rational Design approach, they explore 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. The contributors to the volume then evaluate a set of conjectures in specific issue areas. (This book is Volume 55, part 4 of International Organization.)