The Treaty on European Union, popularly known as the Maastricht Treaty, has had a profound impact on the process of European integration. This book commemorates the entry in force of the Treaty 20 years ago by evaluating the legal impact it has had on the European Union's institutional framework and its policies. It includes contributions on a wide variety of topics, including Economic and Monetary Union, Union citizenship, fundamental rights, foreign affairs and defence policy, educational policy and environmental policy. The different contributions revisit the reasons for the changes the Maastricht Treaty has brought about and examine their development over the years, identifying unexpected success stories, unfulfilled expectations, missed opportunities and future challenges. The contributors are or were associated with the Maastricht Centre for European Law or the Faculty of Law of the Maastricht University, giving this book a unique 'Maastricht' flavour. This book will be of interest to legal scholars and practitioners with a keen interest in EU law and European integration.