The movement toward institutionalization has not escaped the notice of international relations theory. Some of the major theoretical debates in international relations over the last two decades have involved new attempts to understand why states turn to institutions, and what effects these institutions have on patterns of state behavior. The first section of this paper reviews rationalist theories of international institutions, an approach that has its roots in liberal theory but has developed a genuinely new perspective on the role of institutions in world politics. Next, the paper turns to considering some of the major research puzzles in the study of international institutions. While we observe institutionalization, many aspects of this process remain highly puzzling and demand better integration into our explanatory frameworks.