More than 70 years after the Second World War ended, European Union efforts are being challenged by the return of the nation-state. This development raises a number of questions. How should compliance with EU law be ensured if more member states invoke derogations with reference to national sovereignty? Is there a pan-Europeanization of political and economic development in the EU and what does Brexit mean for the development of the EU? How is the distribution of power between the EU and the member states organized and what opportunities do they have for formulating policies that benefit EU citizens? What does it mean that the development of national and EU policies in areas such as migration, taxation, labor market, and regional and structural policies are increasingly being promoted nationally at the expense of the common? How is EU cooperation affected by regional separatism in EU Member States?