What is international legitimacy and whence does it stem? What entities seek it and why, and who grants or withholds it? How might the different meanings of the concept be reconciled? This article argues that Family Resemblance Concept (FRC) methods are particularly well-suited to explicating the complex meanings associated with this multidimensional concept. We start with a basic level definition based on subjective perceptions and beliefs, the normative quality of oughtness, and the idea of consent. We then expand this definition by developing several secondary-level dimensions: shared values, constitutionalism (consisting of two forms of process legitimacy), and outcome legitimacy. At the indicator level, we examine 14 different survey questions asked in international public opinion polls to provide a tentative empirical glimpse of how our FRC version of legitimacy could be operationalised and tested. The paper concludes with a discussion of the usefulness of the FRC scheme in imposing some order on the legitimacy concept and in illuminating the recent legitimacy problems afflicting the United States.