The responsibility of the European Union (EU) and its Member States for breaches of International Investment Protection Agreements to which the EU is a party (EU IIPAs) is a highly topical and, to a large degree, unexplored subject. This is due to recent developments: The entry into force of the Lisbon Treaty in 2009 that shifted treaty-making competences for IIPAs to the EU, the imminent conclusion of the first post-Lisbon EU IIPAs and the adoption of internal EU legislation, which deals both with the management of disputes under EU IIPAs and the internal allocation of financial responsibility flowing from these disputes between the EU and its Member States. The subject is further of significant importance to its stakeholders: Arbitral awards and settlements arising out of disputes under EU IIPAs can and most certainly will, as experience has shown, churn out tremendous sums payable to aggrieved investors. Financial responsibility arising from these disputes can cut into the budgets of both the EU and the Member States depending on who is responsible; financial responsibility of the EU is practically shared by all Member States. A clear, fair and balanced delineation of responsibility between the EU and the Member States for breaches of EU IIPAs is, hence, crucial for a successful post-Lisbon EU international investment policy.