Flexible Integration is a model of reform designed to overcome the current stalemate between federalists and anti-federalists. It introduces more flexibility to accommodate the heterogeneous interests in Europe without risking the gains achieved through past integration. Flexible integration combines firm commitment by all members to a supranational common baseincluding a well-defined set of competences related to the Single Market - with optional integration in other areas through open partnerships.

Based on a theory of differentiated integration, this book investigates the dynamics of flexible European integration across EU policies and member states. By doing so, it provides a theory-based, comprehensive, and an empirical account of European integration from the perspective of legal differentiation.

Differentiation was at first not perceived as a threat to the European project, but rather as a tool to promote further integration. Today, more EU policies than ever are marked by concentric circles of integration and a lack of uniform application. As the EU faces increasingly existential challenges, this timely book considers whether the proliferation of mechanisms of flexibility has contributed to this newly fragile state or whether, to the contrary, differentiation has been fundamental to integration despite the heterogeneity of national interests and priorities.

"The Routledge Handbook of Differentiation in the European Union offers an essential collection of ground-breaking chapters reflecting on the causes and consequences of this complex phenomenon. With contributions from key experts in this sub-field of European Studies, it will become a key volume used for those interested in learning the nuts and bolts of differentiation as a mechanism of (dis)integration in the European Union, especially in the light of Brexit. Organised around five key themes, it offers an authoritative 'encyclopaedia' of differentiation and addresses questions such as:

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.

Despite western Europe's traditional disdain for the United States' "adversarial legalism," the European Union is shifting toward a very similar approach to the law, according to Daniel Kelemen. Coining the term "eurolegalism" to describe the hybrid that is now developing in Europe, he shows how the political and organizational realities of the EU make this shift inevitable. The model of regulatory law that had long predominated in western Europe was more informal and cooperative than its American counterpart. It relied less on lawyers, courts, and private enforcement, and more on opaque networks of bureaucrats and other interests that developed and implemented regulatory policies in concert. European regulators chose flexible, informal means of achieving their objectives, and counted on the courts to challenge their decisions only rarely. Regulation through litigation-central to the U.S. model-was largely absent in Europe. But that changed with the advent of the European Union. Kelemen argues that the EU's fragmented institutional structure and the priority it has put on market integration have generated political incentives and functional pressures that have moved EU policymakers to enact detailed, transparent, judicially enforceable rules-often framed as "rights"-and back them with public enforcement litigation as well as enhanced opportunities for private litigation by individuals, interest groups, and firms.

The book offers a systematic discussion of the facets of the relationship between the European Union and customary international law.