Abstract EU law affects the validity and applicability of intra-EU International Investment Agreements (IIAs) both under EU and international law. Although EU law and intra-EU IIAs create separate legal systems, they cover largely the same subject matter creating scope for potential conflicts. Using different tools and based on different legal principles, both EU law and international law require the determination of the specific incompatibilities between EU law and intra-EU IIAs in order to assess whether and to what extent the latter are applicable. In that regard, the provisions of intra-EU IIAs on dispute settlement and in particular investor-State arbitration present an outright incompatibility with EU law, as they violate the principle of autonomy of EU law and, thus, should be deemed inapplicable.