The present dissertation investigates the controversial issue of intra-EU applicability of the Energy Charter Treaty (ECT) and the compatibility of the investor-State dispute settlement (ISDS) mechanism enshrined in Article 26 ECT with EU law, focusing on the legal developments that have unfolded following the Court of Justice of the European Union (CJEU) landmark ruling in Republic of Moldova v. Komstroy LLC (Case C-741/19). While Slovak Republic v. Achmea ruling (Case C-284/16) laid the conceptual groundwork by declaring intra-EU Bilateral Investment Treaty (BIT) arbitration incompatible with the EU legal order, it is Komstroy that solidified a decisive shift in investment arbitration jurisprudence with direct implications for multilateral agreements such as the ECT. The central research aim of this thesis is to determine how the cumulative legal developments -relevant jurisprudence, the 2020 Termination Agreement, the modernization process of the ECT and the EU’s coordinated withdrawal- have impacted the structural tensions between international investment arbitration and the EU’s principles of autonomy, primacy and exclusive competence. To address this inquiry, the analysis proceeds through three substantive chapters. Chapter 1 sets the legal and institutional foundation. It introduces the ECT and the constitutional principles of EU law -autonomy, primacy and exclusive competencebefore outlining the structural tensions that arise from the ECT’s operation within the EU’s legal order. Chapter 2 analyzes the evolution of intra-EU ISDS jurisprudence following Komstroy, tracing how arbitral tribunals, EU and Member State courts as well as enforcement fora within and beyond the Union, have responded to the growing conflict between intra- EU ISDS and EU constitutional law. It also includes an assessment of post-Brexit enforcement practice before UK courts, which, detached from the CJEU’s authority, continue to uphold the enforceability of ICSID awards under the ECT. Chapter 3 analyzes the EU’s responses to Achmea and Komstroy. It evaluates the EU’s initial attempt to eliminate intra-EU ISDS through the 2020 Termination Agreement. It considers the agreement’s legal foundations under international law, its mechanisms for terminating intra-EU BITs and the unresolved controversies surrounding retroactive consent withdrawal and sunset clauses. The Chapter then turns to the ECT modernization process and the EU’s subsequent withdrawal. It assesses the legal effect of the newly introduced Article 24(3), which excludes intra-EU ISDS, the implications of provisional application and the legal uncertainty surrounding inter se agreements at neutralizing the sunset clause within EU. Through doctrinal legal analysis, treaty interpretation, and policy review, this dissertation argues that Komstroy marks a turning point in the EU’s approach to investment protection. Rather than resolving the intra-EU ISDS dilemma, it initiated a systemic reconfiguration. The result is an incomplete but ongoing transformation of the EU’s investment protection architecture -one marked by disconnection from traditional ISDS and a contested reassertion of constitutional authority.