It is no longer contested that statements of international law issued by various international actors play an important part in shaping the content of international law in a way that cannot be explained by reference to formal legal entitlements. This article argues that the concept of authority is particularly well-suited to account for the normative effects of such statements. Building on rhetoric, sociology and political philosophy, it conceptualizes authority as deference entitlement and identifies four marks of authority to say what the international law is that are not coextensive with the formal power to compel through binding decisions. It then turns to broader implications of this concept of authority for international law and concludes with suggestions for possible further research.