This book offers a legal and political theory of the European Union. Many political and legal philosophers compare the EU to a federal union. They believe that its basic laws should be subject to the standards of constitutional law. They thus find it lacking or incomplete. This book offers a rival theory. If one looks more closely at the treaties and the precedents of the European courts, one sees that the substance of EU law is international, not constitutional. Just like international law, it applies primarily to the relations between states. It binds domestic institutions directly only when the local constitutions allow it. The member states have democratically chosen to adapt their constitutional arrangements in order to share legislative and executive powers with their partners. The legal architecture of the European Union is thus best understood under a theory of dualism and not pluralism. According to this internationalist view, EU law is part of the law of nations and its distinction from domestic law is a matter of substance, not form. This arrangement is supported by a cosmopolitan theory of international justice, which we may call progressive internationalism. The EU is a union of democratic peoples, that freely organize their interdependence on the basis of principles of equality and reciprocity. Its central principles are not the principles of a constitution, but cosmopolitan principles of accountability, liberty, and fairness,