International institutions play important roles in political-military issues as well as in economic and environmental affairs. Indeed, it is impossible to understand efforts to resolve regional and local conflicts, or the form and pace of alliance formation and expansion, without payingattention to security institutions. Imperfect Unions discusses a wide variety of security institutions, including NATO, the Western European Union, United Nations peacekeeping, the ASEAB Regional Forum, and the Organisation for Security and Co-operation in Europe. It describes changes in security institutions, documents the effectsof such institutions on national policies, and explores the conditions that affect the patterns of co-operation and discord that ensue. The book helps to improve our understanding of recent developments in international relations such as NATO enlargement and the regionalization of peacekeeping. In theoretical terms, it shows how institutionalist approaches, such as those represented in this volume, can enrich the important fieldof security studies.