Representativeness has become a keyword in reform projects of global governance. While it often appears as a cure for the legitimacy deficit of international organizations, this notion has hardly been defined at the global level. This article aims at filling this void by providing a socio-historical account of the meanings and uses of this concept based on a comparison of the International Labor Organization (ILO) and the International Organization for Standardization (ISO), two of the oldest organizations in the global arena. Looking at the genesis of both organizations, the article shows that the categories and mechanisms used by international organizations to select their members are embedded in a broader socio-historical, political, and economic context. Moreover, they are subject to contestation and transformation. Representativeness may therefore be used both as a political resource for international players and as an analytical category for the study of the multilateral system over time.