There is much experimental data which indicates that ionizing radiation is a very potent carcinogenic agent. Most types of carcinoma can be produced by radiation. Carcinoma is apparently induced through a single or a series of mutations in somatic cells. Radiologists have excess leukemia and other malignancy from external x-ray; uranium and other miners have excess lung cancer from internal alpha radiation; luminous dial painters have excess osteogenic sarcomas; and uranium mill workers appear to have excess lymphomas. A large number of persons are now exposed occupationally to radiation from nuclear reactors, and from various uses of radioisotopes. For the induction of most types of cancers from radiation it appears that the risk is between 0.5 and 2 cancers per rem per million person years. Epidemiological techniques are essential in determining risks of this low magnitude. Other agents may inhibit or enhance the carcinogenicity of radiation.