By arguing that an interdisciplinary approach must be chosen whenever an international phenomenon touches on separate disciplines, this article illustrates such a necessity by using the case of the analysis of the evolving codification of foreign investors’ responsibilities. A literature review demonstrates that the analysis of this phenomenon necessitates taking into account the content and the application of international instruments as well as relations of power between the actors involved. Given that such explanatory factors are respectively the objects of analysis of International Law and International Relations, this analysis requires an interdisciplinary approach and ultimately contributes to consolidate the relevance of International Studies.