What is meant when politicians and lawyers talk of an opt-out, of ""multi-speed"" Europe, of variable geometry and even of European Union ""a la carte""? Will closer co-operation be authorized, and where? These and many other questions are addressed in this work, which deals with the intriguing and controversial development of increased differentiation in European Union law. Adopting a law in context approach, it offers an analysis of differentiation from the Treaty of Rome to the present, including the 1996 Intergovernmental Conference and the Treaty of Amsterdam, a categorization of differentiation and a search for the various causes and objectives of differentiation and its consequences for the future of the European integration process. Particularly relevant in view of the ratification of the Treaty of Amsterdam, legal scholars and political scientists should find this book helpful in keeping abreast of the debates on European constitutional law.