EU law was initially intended to apply fully to all EU Member States. Increasingly, however, many aspects of EU law apply to only some of those Member States. This book explores this new paradigm of the 'unravelling' of EU law, examining in particular how this applies in the areas of economic and monetary union and justice and home affairs law, and how the general rules on 'enhanced cooperation' within the EU legal system are being applied and should be interpreted. It also analyses the renewed tendency of groups of Member States to negotiate treaties between them which are formally outside the EU legal order but which are nevertheless closely linked to that legal order - such as the recent treaties concerning financial assistance and fiscal stability which aim to ensure the survival of the single currency. The book also assesses the unravelling of the EU's legal order by comparison with the fragmentation of international law and the development of asymmetrical federalism. Finally, the book examines the future of the European Union in light of these developments. Should they be further encouraged, in particular in order to satisfy British calls for renegotiation of the terms of its EU membership? What could and should be the final shape of the unravelled EU legal order?