This thesis develops a theory of international social closure to examine (i) the politics of membership in status groups – or, clubs – in international society and (ii) the persistence of clubs in international society. This thesis offers new concepts to improve the English School’s understanding of international society, its expansion, and its reproduction. In so doing it also addresses limitations and gaps in the IR status literature and the global governance and diplomacy literatures concerned with clubs and networks. This thesis analyses strategies of exclusion, entry, and incorporation used by actors to deny, attempt, or grant inclusion into clubs as well as the institutional contexts underpinning those clubs. Specifically, this research undertakes a study of instances of exclusion, entry, and incorporation in the context of three clubs: the Family of Civilised Nations, the Great Powers club, and G-summitry. In the first two cases, this research relies primarily on secondary sources while in the case of G-summitry it presents original empirical research gathered through archival research, interviews, and ethnographic participant observation. This thesis presents four main conclusions about the operation of closure: (i) the logics of different closure games are defined by overarching normative institutions of international society; (ii) despite a collectivist closure rule, closure in international society is predominantly individualistic; (iii) actors seeking entry tend to employ deferential entry strategies that reproduce a stratified status quo order; and (iv) incorporation promotes stratification along both functional and cultural lines. This thesis also draws three specific conclusions that run counter to much current scholarship: (i) contemporary international society is neither more open nor less hierarchical than nineteenth century international society; (ii) hierarchy is reproduced to a large degree by entry and incorporation strategies rather than exclusion strategies; and (iii) closure does not run along a ‘west versus the rest’ fault line.