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 authorised and where? These and many other questions are addressed in this new book, which deals with the intriguing and controversial development of increased differentiation in European Union law. Adopting a law in context approach, the book offers an analysis of differentiation from the Treaty of Rome to the present, including the 1996 Intergovernmental Conference and the Treaty of Amsterdam, a categorisation 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 will find this book invaluable to keep abreast of the present debates on European constitutional law.