This master's thesis explores the future of international investment law in the European Union with an emphasis on the investment agreements concluded between EU Member States. The majority of them consists of bilateral investment agreements (intra-EU BITs). Ever since their number increased significantly after the enlargement of the EU in 2004, the European Commission claims they are incompatible with EU law and is joined by some Member States that presented the alleged incompatibility as a jurisdictional objection before the investment tribunals hearing disputes brought by investors against them under intra-EU BITs. Aside from intra-EU BITs, similar claims were made in the context of the Energy Charter Treaty (ECT) concluded between Member States as well as third countries, which covers foreign investments in the energy field. In March 2018, the compatibility of intra-EU BITs in the context of the arbitration proceedings between the Dutch investor Achmea and Slovakia was considered by the Court of the EU which found an incompatibility between the arbitration clause contained in the BIT between the Netherlands and Slovakia and the judicial system of the EU as well as the autonomy of EU law. While most of the theoretical discussion is focused on the consequences of the decision from the standpoint of EU law, this thesis assesses the relationship between the Treaty on the Functioning of the EU (TFEU) and the intra-EU BITs as well as the ECT respectively from the standpoint of international law, i.e. as a relationship between treaties with an emphasis on the provisions of the Vienna Convention on the Law of Treaties while also taking into account the arbitration awards available so far. In the context of this assessment, the impact and reach of the decision of the Court of the EU is also considered. The international law analysis of the relationship between the TFEU and intra-EU BITs as well as the ECT respectively shows that the intra-EU BITs, including their arbitration clauses, remain valid and applicable while the ECT also remains applicable in the intra-EU context which is true even after the judgement in Achmea. On the other hand, the political commitment of the Member States to terminate all intra-EU BITs still suggests the end of intra-EU arbitration based on them whereas the fate of the ECT remains more uncertain.