The Sources of International Investment Law; the Primacy of National Legislations

International Investment Law is generally recognized as a law system among the subsystems or the specialized systems of Public International Law. Regardless of this system’s nature, its norms and rules have been applied to the governments and the investors (incl. individuals and corporations) for many years. However, there is still no consensus among the scholars regarding the sources of this system; some define these sources in light of the established sources under Article 38 of the Statute of ICJ, and others draw a different set, albeit with some variations. However, the sources of this system have not yet been reasonably and scientifically explained. To achieve such an apprehension, this article attempts to answer the question “What are the sources of international investment law, and how they have contributed to the evolution of the rules of the system?”. Therefore, through a descriptive-analytical method and in qualitative research nature, this article pursues the assumption that the national legislations, treaties, international customary law, and the general principles of law are the primary sources in this field; subsidiary sources are namely judicial decisions and legal teachings. In this regard, the present article examines its assumption after providing an introduction, research background, theoretical foundations, and a brief history of the concept; Eventually, it is concluded that the national legislations are by far the most important source of international investment law, and we can safely say that other primary and subsidiary sources indeed play an important, though a supplementary role for these legislations.