The design of international institutions varies in many ways: Institutions can be more or less formal, flexible, independent, precise, inclusive, centralized, and so on. This article classifies theoretical efforts to make sense of these similarities and differences. First, some theories focus on the bargains or contracts that attempt to construct equilibrium behavior while other theories analyze institutional design as a dynamic process. Second, theories vary in whether they understand institutional design as a response to the environment in which institutions operate or as a function of the incentives, interests, values, initiatives, and power of the actors that created the institutions. The article discusses four ideal-typical theoretical approaches that fit in each quadrant of the resulting 2×2 typology: rational functionalist, distributive rationalist, historical institutionalist, and structural process theories. These approaches identify different causes for suboptimal or even dysfunctional institutional design: domestic politics, power politics, path dependence, and culture. The conclusion discusses how these theories can help us make sense of current challenges to institutions and their design.