Currently the EU is facing some fundamental challenges: the euro and refugee crises are still unresolved, the solidarity between Member States is eroding and nationalist forces are growing as support for European integration is diminishing. The union appears too heterogeneous and the economic and political interests of the 28 Member States too diverse to be able to address these challenges as a whole. Does the concept of differentiated integration offer a possible solution? Can such differentiation offer the European integration project a better and more stable future? Which measures of diversity and flexibility can the “ever closer Union among the peoples of Europe” endure? Or will such differentiation ultimately damage the joint European project? This collection of leading European lawyers tackles these questions and suggests how they might be addressed.