While legitimacy dynamics are paramount in global governance, they have been insufficiently recognized, conceptualized, and explained in standard accounts of international cooperation. This special issue aims to advance the empirical study of legitimacy and legitimation in global governance. It engages with the question of when, how, and why international organizations (IOs) gain, sustain, and lose legitimacy in world politics. In this introduction, we first conceptualize legitimacy as the belief that an IO's authority is appropriately exercised, and legitimation and delegitimation as processes of justification and contestation intended to shape such beliefs. We then discuss sources of variation in legitimation processes and legitimacy beliefs, with a particular focus on the authority, procedures, and performances of IOs. Finally, we describe the methods used to empirically study legitimacy and legitimation, preview the articles of the special issue, and chart next steps for this research agenda.