Includes supplementary material for the online appendixWe explore how “international organizations” have been conceptualized and operationalized in the field of International Relations (IR), identify an important gap between the two, and demonstrate how this shapes our understanding of world politics. Traditionally, we show, IR has embraced a broad conception of international organizations (IOs) that appreciates variation in design. However, the literature has largely coalesced around a measurement standard that reflects the characteristics of major postwar IOs. Prevailing measures, therefore, mainly count formal IOs—bodies founded with legally binding agreements—and omit informal IOs, which are founded with non-binding instruments. We argu...