Our understanding of international organizations depends on the perspective from which we view them: outside-in or inside-out. The choice of perspective is critical, as each approach relies on different assumptions, provokes different questions and research agendas, and suggests different roles for the organization. This chapter reviews international organizations from these two perspectives. Each approach provides insights that the other lacks. And though the externalist and internalist perspectives are not necessarily exclusive, distinguishing the two, and their variants, provides a guide to a complex literature. Doing so also allows us to track these arguments over time. The resonance of external and internal approaches has changed as the roles, powers and influences of organizations have expanded. At the same time, the disciplinary rigidity and dichotomies that marked earlier eras appear to have subsided in favour of eclecticism, eliding the disjunctive and categorical thinking that has marked the study of international organizations.