In order to understand the reform of international investment law envisioned by the EU, the author provides a comprehensive but concise analysis of the EU reform approaches, its constitutional and legal framework, the concepts of the rule of law and legitimacy, and the reasons for the reform. In particular, the book exposes tensions between the EU aspiration to enhance the rule of law in international investment law, as a means of legitimising this legal discipline, and the challenges of its reform approaches in practice. The analysis combines substantive and procedural aspects of the EU reform of international investment law in the intra-EU context and EU external relations. This book thus critically evaluates the EU vision of the rule of law in international law and its contribution to the development of international law in the field of investment.