According to its Constitution, the mission of the World Health Organization (WHO) was nothing less than the ‘attainment by all peoples of the highest possible level of health’ without distinction of race, religion, political belief, economic status, or social condition. But how consistently and how well has the WHO pursued this mission since 1946? This comprehensive and engaging new history explores these questions by looking at its origins and its institutional antecedents, while also considering its contemporary and future roles. It examines how the WHO was shaped by the particular environments of the postwar period and the Cold War, the relative influence of the US and other approaches to healthcare, and its place alongside sometimes competing international bodies such as UNICEF, the World Bank, and the Gates Foundation. The authors re-evaluate the relative success and failure of critical WHO campaigns, from early malaria and smallpox eradication programs to struggles with Ebola today.