Far from displaying a uniform pattern, European integration varies significantly across policy areas and individual countries. Why do some member states choose to opt out of specific EU policies? Why are some policies deeply integrated whereas others remain intergovernmental? In this updated second edition, the authors introduce the most important theoretical approaches to European integration and apply these to the trajectories of key EU policy areas. Arguing that no single theory offers a completely convincing explanation of integration and differentiation in the EU, 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.