This chapter traces how population came to be seen as a global problem over the twentieth century and into the present. It introduces the neo-Malthusians, including Radhakamal Mukerjee, Warren Thompson and Margaret Sanger, with their vision of a world in which reduced fertility and migration from overcrowded to under-utilized lands would free up food for all. It then focuses on the uptake of population matters within the League of Nations, especially International Labor Office and Health Committee, while stressing the caution necessitated by Catholic hostility. Created in 1939, the Demographic Committee relocated to the Princeton Office of Population Research. Within the United Nations, population activists took influential positions, but Catholic and Communist opposition initially stopped reproductive control becoming part of any technical assistance programme. US-led philanthropic foundations, notably the Population Council, did much to change the international conversation, but population control was widely discredited by encounters of state power with citizens’ bodies in the 1970s and 1980s. The feminist argument for reproductive rights trumped geopolitical and food-security arguments for family planning, but the successor NGOs remain active in ‘reproductive health’ and ‘sexual health’. Climate change has put population and environmental limits back on the agenda again.