This chapter examines the role of authority in public international law, based on the preceding discussion of authority and legitimacy in the context of exercises of power. Although there is a source of power, if not authority, at the domestic level in the form of government, this does not automatically transfer to the international level. This is primarily because the international system exists without a central authority. The question automatically arises of who ought to be able to make such rules, and whether the requirement for the exercise of autonomy is still as critical as it would be at the domestic level. The discussion is based on an analysis of foundational texts, exploring the ideas of authority at the international level expounded by Grotius and Vitoria, before moving on to work by more recent authors.