In this chapter we examine how international investment law in relations between the Member States poses a challenge to EU law and its autonomy, hindering internal market integration as one of the core EU objectives, and the rule of law in the EU as its core value. We analyse tensions between international investment law and EU law, focusing first on intra-EU international investment agreements and the objectives of EU integration, and second on intra-EU investment treaty arbitration and the autonomy of the EU legal order. The Achmea judgment of the CJEU is the apotheosis of these tensions, and it has been endorsed in subsequent CJEU judgments. In the final part of this chapter, we analyse legal and policy implications of this jurisprudence, in the broader context of EU integration and the rule of law in the EU.