The Maastricht Treaty, signed in 1992 and ratified in the following year, is widely seen as a landmark in the evolution of the European Union. It introduced into the treaty framework revolutionary new elements such as the co-decision procedure between the Council and the European Parliament, cooperation in the area of Justice and Home Affairs, the Common Foreign and Security Policy and the "euro" as a single currency for the majority of the then member states. It also introduced the concept of European citizenship into the treaty, reflecting the rising expectations of both citizens and decision-makers in the European project, and upgraded the role of the European Council at the summit of the EU's institutional structure. Twenty years later, each of these innovations remain of central importance for the process of European integration, while current developments provide a valuable opportunity to reflect on the historical decisions taken in Maastricht in order to assess their significance and examine the subsequent evolution of the Union. This volume brings together an international group of leading scholars in the field in order to provide such an assessment, with each article both looking back over the developments within each of these domains as well as looking ahead to the way in which the EU is positioned to address current challenges. This book was published as a special issue of the Journal of European Integration.