While European leaders have worked on compromises and bargains over the years, political scientists have produced a bewildering array of conceptual frameworks on European integration: intergovernmentalism, functionalism, transnationalism, neoinstitutionalism, and many more. This book focuses on the theory of European integration rather than its history and practices, but it tends to leave the reader bewildered about the gap between the complex and fluctuating reality of integration and the theoretical analyses that often leave it far behind. All the same, Rosamond offers a reliable guide through the jungle of theories in the field. He tries to explain how a strange new animal -- the European Union -- fits within the universe of traditional international-relations theory, which views the state as the main actor and international regimes as centers of interstate cooperation. In this framework, the EU is seen as a puzzle because it not a federation but is more than a regime; moreover, it clearly reduces the sovereignty of its member states. What nonspecialists will make of all of this is hard to guess, but Rosamond remains lucid, fair, and crisp throughout.