What does authority mean under international law? There are various actors with different forms of authority, but no overarching concept of what characteristic endows an actor with authority, and even less of a coherent conception of legitimacy as a requirement for such authority. In fact, international law recognizes different authorities for different causes and different contexts, allocated to different actors, who base their authority on different characteristics (state legitimacy, representativity, military power, control). After disaggregating the concept of authority and outlining some of the consequences that follow from each type, this article highlights a number of different actors and describes the various authorities each has under international law. For instance, under jus in bello, nonstate actors can create a state of armed conflict in which they can often continue to use military means without legal sanction. While jus ad bellum may still in principle require legitimacy (in the formal sense of being a state), current jus in bello covers a range of non-state actors. Thus, from a practical point of view, the jus in bello regulations undermine any jus ad bellum requirement of legitimate authority.