By remaining epistemologically committed to an outdated distinction between facts and values, traditional definitions of legitimacy either divest the concept of an important critical component or fail to properly anchor it in the social reality. As an alternative, the article proposes a constitutive theory of legitimacy that reconciles the epistemological tension via the concepts of fairness and tractability and bridges the theoretical gap via the concept of deliberative legitimacy. By applying it to the issue of the use of force, the article argues that a normatively sound and politically relevant model of legitimacy must take into account both to the manner in which actors’ commitment to social norms is crafted and the type of conditions that facilitate or constrain the definition, contestation, and adjudication of what counts as legitimacy in a particular political context.