How do international organizations (IOs) claim legitimacy, and why do they do so in different ways? Confronted with contestation and critique, IOs seek to enhance audiences' beliefs in their legitimacy by justifying their governance competence through public communication and the change of institutions and behaviour. This article serves as an introduction to a special section on 'Legitimizing international organizations'. It theorizes the origins of IOs' legitimation strategies and outlines the analytical framework for the special section. We propose the agents-audiences-environment (AAE) framework, which synthesizes diverse literatures on organizational legitimation. While existing literature focuses on audiencesapos; normative demands as a key source of legitimation strategies, we supplement this perspective with ones that consider IO agents' normative beliefs and the norms institutionalized in peer organizations. In this introduction, we first clarify what is at stake in the debate over IO legitimation. We then explain the benefits of shifting perspective from audience beliefs to the origins of IO legitimation. Thereafter, we define the main concepts and develop our AAE-framework. We conclude by summarizing how our contributors use the AAE-framework to advance our understanding of IO legitimation.