This book explores one of the central challenges facing the EU today – how to reconcile enlargement with the pursuit of a stronger and more effective European Union. While the relationship between widening and deepening has been recognized for years as one of the big questions in the field of European integration, existing theoretical and empirical analyses of this relationship suffer from a variety of shortcomings. This book brings together a group of EU scholars who significantly advance our understanding of the relationship between widening and deepening. The contributors challenge a variety of ‘common wisdoms’ concerning the relationship between widening and deepening and offer nuanced theoretical and empirical analysis of the relationship between these two vital dimensions of European integration. Collectively, the contributors to this volume offer the most comprehensive picture available to date of the multi-faceted relationship between widening and deepening. This book was published as a special issue of the Journal of European Public Policy.