Joseph Rotblat not only believed that the purpose of science was to serve humanity; he also lived by his beliefs. Although there were a few leading physicists, including Max Born and Lise Meitner, who refused to have anything to do with the wartime atomic bomb project, Rotblat was the only scientist who left Los Alamos, late in 1944, when he discovered that there was no longer any danger of a Nazi bomb. Following the destruction of Hiroshima and Nagasaki in 1945, Robert Oppenheimer, the scientific director of the Manhattan Project, became world famous and an international hero. After the war Oppenheimer worked publicly to regain control of nuclear weapons, while privately endorsing the development of the H-bomb on the grounds that it was ‘technically sweet’. Rotblat, by contrast, abandoned nuclear physics and became the first professor of medical physics at St Bartholomew’s Hospital in London. In a way, Rotblat was everything that Oppenheimer was said to be but was not.