The steadily rising number of investor-State arbitration proceedings within the EU has triggered an extensive backlash and an increased questioning of the international investment law regime by different Member States as well as the EU Commission. This has resulted in the EU’s assertion of control over the intra-EU investment regime by promoting the termination of bilateral intra-EU investment treaties (intra-EU BITs) and by opposing the jurisdiction of arbitral tribunals in intra-EU investor-State arbitration proceedings. Against the backdrop of the landmark Achmea decision of the European Court of Justice, the book offers an in-depth analysis of the interplay of international investment law and the law of the European Union with regard to intra-EU investments, i.e. investments undertaken by an investor from one EU Member State within the territory of another EU Member State. It specifically analyses the conflict between the two investment protection regimes applicable within the EU with a particular emphasis on the compatibility of the international legal instruments with the law of the European Union. The book thereby addresses the more general question of the relationship between EU law and international law and offers a conceptual framework of intra-European investment protection based on the analysis of all intra-EU BITs, the Energy Charter Treaty and EU law, as well as the arbitral practice in over 180 intra-EU investor-State arbitration proceedings. Finally, the book develops possible solutions to reconcile the international legal standards of protection with the regionalized transnational law of the European Union.