The new competence of the EU in the field of international investments, gained with the Lisbon reform has determined a phase of turmoil in the existing system of international governance of foreign investments both at EU and international level. In the view of the Authors, the EU impetus in seeking innovative solutions is connected to, if not compelled by, the inherently complex relationship between EU law, on one side, and international investment and arbitration law, on the other side. The paper aims to provide an overview of the most controversial issues arising out of the interplay between EU law and investment arbitration, with the view to assessing whether the current developments taking place at EU level are capable of solving or at least mitigating them. Some general principles of law are used as a toolbox of the analysis i.e. pacta sunt servanda, the principle of legal certainty, the principle of sincere cooperation and that of autonomy of EU legal order. The first and second principles are common to both EU law and international (investment and arbitration) law, whereas the third and fourth ones are inherent only to the EU legal order; however, all of them serve the goal to show the difficulties the EU has in trying to build a coherent system between the two subject matters.