The European Union has more and more far-reaching competences in external relations than ever before. It is legally committed to a long list of ambitious and comprehensive foreign policy objectives. Arguably, the Union’s foreign policy objectives (Article 21 TEU) are more ambitious and comprehensive than the foreign policy objectives set out in the national constitutions of the Member States. At the same time, Member States have in recent years challenged Union external actions in an unprecedented number of cases, including in order to reign in Union action and constrain Union powers. On the one hand, in an ever more globalised world it is rational for the EU Member States to confer powers to the Union in order to have jointly a greater say of how global challenges are tackled and what role the Union and its Member States are playing internationally. On the other hand, Member States are ever more heterogeneous in their interests and views of (how to tackle) these challenges. This edited collection unearths and reflects on some of these tensions, including in the areas of the environment, migration and Brexit.