In contrast to the ‘ever closer Union among the peoples of Europe’ invoked in the preamble of the Treaty of Rome, Frank Schimmelfennig and Thomas Winzen ponder whether the European Union (EU) is ‘growing ever looser’. Differentiation can take various forms which are often intertwined, such as functional, institutional, spatial/territorial as well as temporal differentiation. In 1998, Petersen studied Denmark’s integration policy in what he called a ‘dilemma’ between influence capability and stress sensitivity. With the Great Recession of 2007This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book 2008, the EU entered a new multi-faceted crisis. The future of European integration became an increasingly debated issue, and so did the issue of differentiated integration. The relevance of differentiation beyond the Brexit vote is further exemplified by the European Commission’s publication of its ‘White Paper on the Future of Europe’, which established several concrete scenarios for Europe by 2025. The conclusion reflects on the White Paper’s contribution to debates over the future of Europe following the 2019 European Parliament elections.