The law of the external relations of the European Union is a subject of great importance. The EU institutions have developed an extensive practice in this area, by concluding international agreements, by participating in the work of international organizations, and by legislating and regulating on matters of external relations. It is a practice giving rise to many legal problems and questions, as evidenced by the substantial and growing body of case-law in the area of external relations by the European Court of Justice and Court of First Instance. These problems and questions are often of constitutional significance, and the external relations law of the EU therefore occupies an important position in the overall development of the EU, and in the process of its constitution-building. This volume examines the legal and constitutional foundations of the EU's external relations. It focuses on the EU's external powers and objectives, on the instruments, principles, and actors of external policies, and on the legal effects of international agreements and international law. It brings together the legal foundations of the EU's external economic policies, centred around its trade policy, and the fledgling Common Foreign and Security Policy. It concludes by addressing the basic questions of constitutionalism raised by EU external relations, i.e. the division of competences, the protection of fundamental rights, issues of democracy and legitimacy, and judicial review.