Secession is a live issue in today's Western Europe. In the last decade, we have witnessed the consolidation of pro-independence movements in Scotland and Catalonia and in the near future, we might see their re-emergence or the rise of other pro-secession movements in other European regions. The response of the EU institutions to secession within EU Member States may well be based mainly on political considerations. However, since the EU is a community based on the rule of law, it has also to justify its position with normative arguments of principle. Secession and European Union Law provides such normative support, drawing on a pluralist reading of the relation between EU law and national law, to support the conclusion that EU law should respect domestic constitutional orders. This book studies secession within EU Member States through legal methodology: the theoretical-doctrinal analysis of concepts and institutions, considering the evolving reality and case law. The legal approach has three dimensions, given the three different legal orders that interact at the EU level: international law, EU law and national constitutional law. Based on Article 4 (2) TEU, the central claim of this book is that the EU duty to respect national identity and fundamental constitutional structures generate obligations to respect Member States' constitutional orders, provided that the values enshrined in the Article are not violated by the Member State affected. Topical and original, Secession and European Union Law reviews and rethinks key features of the EU and the EU legal order.