The volume explores the marked differences between the complex and rapidly changing legal organization of EU external relations and the EU's 'internal' constitutional order. The European Union is unique as a polity organized along federal lines but with fully fledged States as its component political entities. The tension between the self-conscious Member States and their constitutional relationship within the EU is especially pronounced in the foreign policy field, where they remain determined to assert their status as full subjects of the international order. This book explores how foreign policy fits within the constitutional structure of the EU, characterized by the division of external relations competences between the EU and the Member States ('the vertical axis'), and between the 'pillars' of which the Union is composed, in particular the division between the Community competences of the first pillar and the common foreign and security competences of the second pillar ('the horizontal axis'). This is a study of the extent to which foreign policy is legally sui generis within the sui generis constitutional order of the EU, and of how the common foreign and security policy is in turn sui generis within the foreign policy structure of the Union. It provides both an exploration of the constitutional reality of EU foreign policy and theoretical analysis which suggests possibilities for reform.