Twenty-four years after the Treaty of Maastricht, the legal and political concept of European Union citizenship remains as important, and yet as contested, as it then was. Two decades of jurisprudence from the Court of Justice, at first expanding the concept and its contingent rights and then rowing back on some of its previous decisions in the field, combined with extensive changes to EU legislation on free movement and migration more generally, have led to extensive and sometimes groundbreaking studies by leading EU scholars and practitioners. Many of them contribute to this book, making it both a leading work on European Union citizenship and a timely reminder of how complex the legal issues raised by this innovative status are as free movement itself becomes a point of contention within the EU.