The author attempts to answer the following question: in an international legal system of which States are no longer the only subjects, what is the role that the consent of IOs plays in the creation of rights and obligations that apply on the international plane? The chapter reflects on the legal agency that IOs enjoy, qua subjects of international law, in the process of formation of general international law and in the acquisition of treaty rights and obligations. In so doing, it advances two claims. The first is that even though IO practice can and should be taken into account in the identification of customary rules, it is not a necessary element in the process of formation of those rules. The second claim of the chapter is that, under current law, IOs can only be bound by treaty rules to which they have consented, which may give rise to problematic gaps between the treaty obligations of IOs and the treaty obligations of their members. In pursuing those claims, the chapter offers some reflections on the systemic and normative implications of the formal involvement of IOs in the making of rules of general and conventional international law.