The notion of Differentiated Integration is increasingly used in the literature on European integration. Often employed interchangeably with the notion of "flexible integration, diverging views on its nature have led to the emergence of various definitions and, to some extent, a semantic confusion. A lack of consensus characterizes the academic literature; some authors even avoid putting an explicit definition on the term. The main objective of this book is to seek answers for the following questions: How can one define Differentiated Integration in the European Union? Should Differentiated Integration be considered as a process, a concept, a system or a theory? Should it be seen as a temporary or a well-established phenomenon? How is this field of study likely to develop in the future? In order to do so, all chapters, written by leading experts in the field, offer a state-of-the-art analysis of the study of differentiated integration, from theoretical and practical perspectives. In addition, this book is not a collection of isolated papers: all chapters are interconnected and gravitate towards the aforementioned central questions, but approach these from different perspectives. This book was published as a special issue of the Journal of European Public Policy.