Far from displaying a uniform pattern of integration, the European Union varies significantly across policy areas, institutional development and individual countries. Why do some policies such as the Single Market attract non-EU member states, while some member states choose to opt out of other EU policies? In answering these questions, this innovative new text provides a state-of-the-art introduction to the study of European integration. The authors introduce the most important theories of European integration and apply these to the trajectories of key EU policy areas - including the single market, monetary policy, foreign and security policy, and justice and home affairs. Arguing that no single theory offers a completely convincing explanation of integration and differentiation in the EU, the authors put forward a new analytical perspective for describing and explaining the institutions and policies of the EU and their development over time. Written by a team of prominent scholars in the field, this thought-provoking book provides a new synthesis of integration theory and an original way of thinking about what the EU is and how it works.