This introductory chapter of the third edition of European Integration Theory (OUP 2018) addresses the rationale for a book on European integration theory and introduces the contributions to the book. It begins by addressing the question of Why Study Integration Theory; it then defines the terms ‘integration’ and ‘theory’ and introduces the ‘mosaic of European integration theory’ as the book’s central concept. The chapter also offers an overview of European integration as a process which has been studied for several decades now. To that end, the chapter recalls distinct phases of integration and the respective parts of the mosaic which have been developed to understand and explain them based on descriptive, analytical and constructive theorising. Each phase is distinguished by historical context, leading questions and relevant theoretical reference points. The book’s extensive section on Studying European Integration by taking account of ‘contexts of theoretical development’ and addressing the question of ‘competing or complementary theoretical approaches’ which also identifies the functions and areas of theory. In concluding, the chapter details the concept of the ‘Mosaic of Integration Theory’ and introduces the chapter structure of the book’s contributions.