Many individuals are, or have been, exposed to ionising radiation in the course of their work and the epidemiological study of occupationally irradiated groups offers an important opportunity to complement the estimates of risks to health resulting from exposure to radiation that are obtained from other populations, such as the Japanese survivors of the atomic bombings of Hiroshima and Nagasaki in 1945. Moreover, workplace exposure to radiation usually involves irradiation conditions that are of direct relevance to the principal concern of radiological protection: protracted exposure to low level radiation. Further, some workers have been exposed to radioactive material that has been inadvertently taken into the body, and the study of these groups leads to risk estimates derived directly from the experience of those irradiated by these 'internal emitters', intakes of alpha-particle-emitters being of particular interest. Workforces that have been the subject of epidemiological study include medical staff, aircrews, radium dial luminisers, underground hard-rock miners, Chernobyl clean-up workers, nuclear weapons test participants and nuclear industry workers. The first solid epidemiological evidence of the stochastic effects of irradiation came from a study of occupational exposure to medical x-rays that was reported in 1944, which demonstrated a large excess risk of leukaemia among US radiologists; but the general lack of dose records for early medical staff who tended to experience the highest exposures hampers the derivation of risks per unit dose received by medical workers. The instrument dial luminisers who inadvertently ingested large amounts of radium-based paint and underground hard-rock miners who inhaled large quantities of radon and its decay products suffered markedly raised excess risks of, respectively, bone and lung cancers; the miner studies have provided standard risk estimates for radon-induced lung cancer. The large numbers of nuclear industry workers around the world present a possibility of deriving risk coefficients of direct relevance to radiological protection, and the recently published study of workers from 15 countries illustrates what can be achieved by international collaboration. However, it would appear that there are some problems with this study that require attention before reliance can be placed upon the results. Early workers from the Mayak plutonium production facility in Russia were heavily exposed to external sources of penetrating radiation and to plutonium, and appreciable effort has been expended in obtaining dependable risk estimates from this scientifically valuable group of workers. Those occupationally exposed to low levels of radiation also present an opportunity of studying possible somatic health effects other than cancer, such as heart disease and eye cataracts, that are the subject of much discussion at present. Overall, studies of exposure to ionising radiation in the workplace provide a valuable support to studies of those groups exposed under other circumstances, and in some instances (such as exposure to plutonium) effectively offer the only direct source of epidemiological evidence on risks.