Ionizing radiation is carcinogenic to many, if not most, tissues. Its carcinogenicity varies, however, depending on the tissue exposed, conditions of exposure, genetic background, sex, age of the exposed individual, and other factors. The neoplasms induced by radiation also vary in their types and in their times of onset, depending on the age and sex of the exposed individual. The long induction period for radiation carcinogenesis and the enhancing or inhibiting effects of other agents acting after irradiation imply that the induction of cancer is a multistage process, in keeping with experiments on radiation-induced cell transformation in vitro. The molecular nature of the steps involved in radiation carcinogenesis remains to be fully elucidated, but it is being rapidly explored through advances in somatic cell genetics and molecular biology. The resulting insights will significantly extend epidemiological data in future attempts to estimate the carcinogenic risks of low-level radiation.