The International Law Commission’s Draft articles on the international responsibility of organizations (DARIO) do not distribute international responsibility between the EU and its Member States. The DARIO framework determines whether a particular entity is internationally responsible. Responsibility in a multi-layered structure like the EU, where sovereignty is shared between separate legal persons, raises additional, important questions that fall outside the scope of DARIO. While a state can transfer its competences to an international body, its international obligations are non-transferable. I argue that both the EU and its Member States are independently responsible for joint obligations, such as under mixed agreements. This principle of independent responsibility, however, provides only a partial answer to the question of shared responsibility. That the responsibility of the EU and its Member States can be invoked does not tell us whether the EU, its Member States, or both, will be responsible in a given situation. The complex task of distributing shared responsibility remains. This study finds that in international economic law the focus of international dispute-settlement bodies is not on the responsible party, but on a party best placed to bear responsibility. Remedies for violations of international economic obligations have a restorative rather than corrective purpose. Accordingly, attribution of conduct is largely irrelevant for the distribution of shared responsibility between the EU and its Member States. Instead, shared responsibility typically raises the question of the proper respondent(s). This thesis argues that whether the EU or its Member States, or both, are the proper respondent(s) in a given dispute to a large extent depends on the nature of the treaty regime in question. An extensive analysis of case law under the World Trade Organization (WTO) and the Energy Charter Treaty (ECT) regimes reveals the underlying rationales for the identification of the proper respondent(s). I suggest that the WTO Dispute Settlement Body follows the positive solution test under which the panels address their recommendations to the entity which is necessary to ensure compliance with WTO law. Under the ECT regime, where the primary remedy is compensation, arbitral tribunals focus on the entity which is the proximate cause of the alleged harm. Building on the analysis of the nature of the WTO and the ECT regimes, I explain why the EU is the optimal respondent under the WTO regime, whereas Member States are the primary respondents in investment disputes. The findings of this thesis are then applied to the question of responsibility of the EU and its Member States under new-generation free trade agreements, such as the EU-Canada Comprehensive Economic and Trade Agreement (CETA). I suggest that investment chapters of new-generation treaties modify the usual patterns of the optimal respondents through procedural techniques which allow the EU to assume responsibility on behalf of its Member States.