This article aims to explain the protean nature of the concept of “legitimacy,” arguing that its variability largely stems from denoting a quality of institutions that is both internally complex and sensitive to variations in institutional context. While this institutional-context sensitivity often leads to confusion and miscommunication, it is also what centers the concept’s meaning and use. To better understand legitimacy’s different forms of institutional-context sensitivity, and how they are interconnected, the article shifts from analysis and comparisons of concepts and theories of legitimacy to analysis and comparison of specific legitimacy arguments regarding specific institutions. It introduces a structured framework for analyzing legitimacy claims, beginning with the identification of the institutional level that the argument is directed at. This approach highlights how legitimacy assessments vary across higher and lower institutional levels—a crucial aspect of institutional-context sensitivity that has been underexplored in recent institutional legitimacy literature. The framework, comprising four steps of analysis and two supporting figures, advances our understanding of the complex nature of institutions’ legitimacy and underscores the importance of distinguishing between the legitimacy of an institution and the legitimacy within an institution. Throughout, the article illustrates the framework with examples drawn from scholarly debates on the legitimacy of the International Criminal Court.