Authority is critical in two key senses. First, it is central to framing the ethical debate about the justice or injustice of war, raising questions about the universality of just war and the tradition’s relationship to religion, law, and democracy. Second, who has the legitimate authority to make just-war claims and declare and prosecute war? Such authority has traditionally been located in the sovereign state, but non-state and supra-state claims to legitimate authority have become increasingly important over the last twenty years as the just war tradition has been used to think about multilateral military operations, terrorism, guerrilla warfare, and sub-state violence. The chapters in this collection, organized around these two dimensions, offer a compelling reassessment of the authority issue’s centrality in how we can, do, and ought to think about war in contemporary global politics.