ABSTRACT: Differentiation has been a feature of European integration for more than two decades. Nowadays, more than half of European Union (EU) policies are now implemented in different ways. Recent debates over a potential British exit from the EU revived discussions on the future of European integration, offering a potential case for disintegration. Yet scholars and practitioners still find it difficult to define the notion. The introduction to this collection offers a survey of the literature on differentiated integration, its most recent developments and justifies why the study of differentiation needs to move up the research agenda of European integration. It suggests that studying differentiated integration as a concept, a process, a system and a theory is the minimum needed to understand it. Finally, it demonstrates the necessity to study differentiation as a permanent and ‘normal’ feature of European integration.