The main aim of the book is to analyse the United in Diversity paradigm of the European integration project. With its comparative approach, the book brings together a wide range of academic work on flexible integration to explore the diversities between the various European countries. Interdisciplinary in scope, it aims to explain the flexible forms of integration in Europe in the light of the new political economy. In doing so, a deep insight into the variety and complexity of differentiation and flexibility in European integration is also of added value for policy makers across Europe working in the field of integration.