This unique book explores what subregions are in a European context and what roles they fulfil in relation to the European integration process, exploring how subregional cooperation and integration in Europe largely take place in the shadow of the European integration process. Mats Braun examines how subregions fulfil two specific functions in relation to the European integration process: they compensate for the fact that countries are connected to the European Union in different ways, and they facilitate cooperation in fields where the EU has failed to do so. The book analyses two of the European subregions in detail, the Nordics and the Visegrad, and explores how these groups gain legitimacy. Braun suggests that Nordic cooperation is based on perceptions of shared norms relating, among other things, to peace, while the Visegrad Group has become widely known only in the aftermath of the migration crisis and is now developing a new narrative based on protecting Europe. Elaborating on a theoretical framework based on postfunctionalism, this book will be critical reading for scholars and students of European politics and policy, international relations and regional studies. Its focus on the two case studies will also be beneficial for policy makers and analysts interested in the politics of the Nordic and Visegrad countries.