Introduction -- Perspectives from the International law of responsibility on the joint responsibility regime of the EU and its member states -- International responsibility in mixed agreements: the case of the WTO Agreement -- The responsibility of the Union Polity in the WTO in light of the constitutional framework of principles -- A model for participation of the Union Polity in the DSP and the management of the responsibility concerns -- Summary and conclusions.This work focuses on the EU’s participation in the Dispute Settlement Proceedings (DSP) of the WTO for matters of non-conferred competences. The underlying thesis is that the joint membership of the EU and its Member States is fallacious, in that it could cause the EU to become responsible for violations of the WTO regulations on the part of the Member States. Such fallacies are rooted in the blurred nature of the distribution of powers in the EU polity. In order to tackle the issue of international responsibility, the analysis is based on the facts of a real-world case. Based on the tenets of public international law, the law of mixed agreements and the EU constitutional principles, the book puts forward a model for the EU’s participation in the DSP, and for the reallocation of burdens to the respective responsible entity. This proposition deconstructs the joint responsibility regime and endorses a solution that could address the issue of responsibility in mixed agreements without a declaration of powers.