This work explores how investment arbitral tribunals have accomplished their task of interpreting the different rules of law within the international investment field and what are the interpretative problems that they have faced. It first, analyses how arbitral tribunals have been exposed to the diverse problems of legal interpretation while at the same time the nature and specificities of investment arbitration have influenced the reasoning process of investment arbitrators. Then, it describes how the different rules of law have been interpreted by arbitral tribunals. These rules of law may belong to international law – treaties, customary international law and general principles of law – or they may originate from national laws, contracts and transnational laws. Finally, this research makes an assessment of the interpretative practice of arbitral tribunals. In particular, it suggests that arbitral tribunals’ interpretative reasoning is strongly influenced by the hybrid nature of these tribunals and by the flexibility of the VCLT. Furthermore, it also aims to establish whether the diverse interpretative techniques that have been employed by arbitral tribunals are contributing to strengthening the investment regime or are jeopardizing it. Finally, this third part also evaluates the possible limitations faced by investment arbitral tribunals regarding their interpretative reasoning.