Legitimacy has become a central concern in international law. This article analyses an important aspect of the concept, namely the often-presumed link between legitimacy and the stability of institutions and norms. The explanatory role of legitimacy hinges on the descriptive elements attributed to legitimacy because, only by fixing those elements, a causal link can be established. The article contends that due to its conceptual features legitimacy cannot be circumscribed descriptively, making the tracing of its relationship to the stability of institutions and norms in the international legal order an intractable task. The article suggests that international lawyers should embrace the open-ended nature of legitimacy and focus on its dynamic dimension: legitimation. Legitimacy is treated as a rhetorical tool whereby actors try to pursue certain courses of action. The importance of legitimacy then lies in its employment for the shaping of perceptions with regard to how institutions ought to be.