The topic of this book was identified during the pre-Lisbon era, while I was working on a research project about the legal challenges that regional trade agreements pose for the World Trade Organization (WTO) agreement. Essentially, from the WTO perspective, the European Union (EU) is deemed to be a preferred art of regionalism as long as the EU common market constitutes an advanced state of economic integration. However, the joint membership of the EU and its member states in the WTO agreement triggers significant legal challenges for this latter in terms of the distribution of the international responsibility for violations of WTO disciplines by the EU or its member states in areas of non-conferred competences. In a more practical level, a question arises about the EU sole participation in the dispute settlement proceedings of the WTO for disputes emerging outside the areas of conferred EU powers. This participation of the EU could provoke significant questions of legality related to the assumption of responsibility for wrongful actions committed by the EU member states while acting within their scope of residual competences. This issue constitutes the main research question of the book. The central assumption is that the way in which the international responsibility of the EU and its member states is distributed sits uneasily with conventional modes provided in international agreements in general. As such, this causes a certain degree of uncertainty for other trading partners, which, by not grasping the constitutional nature of the EU polity, rely on a conventional way of management of the international responsibility. The same problem could be conceived also in light of other international mixed agreements, particularly those operating in the area of international economic law.