Differentiation was at first not perceived as a threat to the European project, but rather as a tool to promote further integration. Today, more EU policies than ever are marked by concentric circles of integration and a lack of uniform application. As the EU faces increasingly existential challenges, this timely book considers whether the proliferation of mechanisms of flexibility has contributed to this newly fragile state or whether, to the contrary, differentiation has been fundamental to integration despite the heterogeneity of national interests and priorities.