Concern about the size of the world's population did not begin with the Baby Boomers. Overpopulation as a conceptual problem originated after World War I and was understood as an issue with far-reaching ecological, agricultural, economic, and geopolitical consequences. This study traces the idea of a world population problem as it developed from the 1920s through the 1950s, long before the late-1960s notion of a postwar "population bomb." Drawing on international conference transcripts and oral testimony, the volume reconstructs the twentieth-century discourse on population as an international issue concerned with migration, colonial expansion, sovereignty, and globalization. It connects the genealogy of population discourse to the rise of economically and demographically defined global regions, the characterization of "civilizations" with different standards of living, global attitudes toward "development," and first- and third-world designations.