It is often argued that regional groupings of states are becoming more important in world politics, but it remains puzzling why regions have taken shape in such diªerent ways around the world and how these differences matter. In this pathbreaking book on the logic and diversity of regional cooperation, Acharya and Johnston provide the best available answers yet to these puzzles. They supply a framework for comparing regions, focusing on "institutional design." Discrete chapters present a rich array of insights about institutional variation and cooperation in Europe, Southeast Asia, Latin America, Africa, and the Middle East. Acharya and Johnston offer fascinating discussions of the different "ideologies" of regionalism, noting that developing countries are more intent on using regional cooperation to safeguard state sovereignty than advanced countries, which tend to pursue more integrative regional strategies. The book makes clear that the world's regions are not all following a single, Western-style trajectory; instead, they are evolving in unique ways to cope with distinct geographic, cultural, and geopolitical realities.