The Isotope School was established in 1951 by the British Atomic Energy Research Establishment (AERE) at Harwell following the model of the American Oak Ridge Institute of Nuclear Studies. Until its dissolution in 1967, it played an important role in the expansion of radioisotope techniques in Britain and Western Europe. This paper traces the origin and activities of the Isotope School, and describes the content of its courses and the composition of its audiences both in Britain and abroad. These illustrate the motivations behind the early diffusion of nuclear technology and the importance of Cold War politics in shaping the flows of materials and expertise. In particular, the ban on attendance of Eastern European students at the courses reveals a persistent tension inside the British nuclear programme: the conflict between the drive for disseminating nucleonics and the restrictions forced by national security concerns. © British Society for the History of Science.