This paper discusses the emergence of the concepts of fusion as an energy source and of the controlled fusion reactor. These concepts are shown to have arisen from the bringing together of several different branches of physics, notably nuclear physics, astrophysics, and gas discharge physics, in the period between the two world wars. By the late 1930s, enough information had become available for the possibility of a controlled fusion device to be explored, and a number of physicists seem to have recognized this independently. Such a possibility was the subject of discussion at Los Alamos during and immediately after the war, but the first detailed proposals to be recorded seem to be those of G. P. Thomson and P. C. Thonemann, both working in Britain in the middle to late 1940s. These proposals, which led to the setting up in Britain of the first coherent programme of controlled fusion research, are outlined and discussed.