Since its earliest incarnations, just war theory has included the requirement that war must be initiated and waged by a legitimate authority. However, while recent years have witnessed a remarkable resurgence in interest in just war theory, the authority criterion is largely absent from contemporary discussions. In this paper I aim to show that this is an oversight worth rectifying, by arguing that the authority criterion plays a much more important role within just war theorising than is commonly supposed. As standardly understood, the authority criterion provides a necessary condition for the justification of the resort to war, but has no bearing on the question of permissible conduct in war. In opposition, I argue for an alternative interpretation of the criterion, which attributes to it a fundamental role in assessing this latter question. With this revised interpretation in place, I then demonstrate its advantages by applying it to the practical issue of armed conflicts that are initiated and fought by non-traditional belligerents. While several theorists have recognised that this common feature of modern armed conflict poses a challenge to mainstream just war theory in general—and to the authority criterion in particular—I argue that existing discussions frequently misconstrue the nature of the challenge, since they assume the standard interpretation of the authority requirement and its role within the theory. I then show that the revised interpretation provides a clearer account of both the challenge posed by non-traditional belligerency and the kind of response that it requires.