The risk of developing a tumor of the nervous system in humans is analysed in several studies of populations, exposed to ionising radiation for medical reasons, or exposed to military or occupational radiation. The main data come from series of patients who underwent radiotherapy during childhood: a high incidence of tumors of the nervous system is found after irradiation of one to a few grays as treatment of a benign disease (especially tinea capitis), as well as after irradiation at higher doses of a few tens of grays for the treatment of cancer (in particular cerebral irradiation in acute lymphoblastic leukaemia). The type of radiation-induced tumors is variable, but meningioma is more frequent after low doses and glioma and sarcoma after higher doses used in the treatment of neoplastic diseases. A dose-effect relationship appeared between the risk of tumor of the nervous system and the radiation dose. The risk was higher when radiation was delivered at a younger age. Much less data are available after radiotherapy in the adulthood, but an increased risk of cerebral tumor appears in the series of ankylosing spondylitis patients. As for the exposures to radiodiagnosis exams, the main problem is the risk of cerebral tumor in children whose mother has undergone abdominal or pelvic X-rays during pregnancy. No risk of neurologic tumor was found in the A-bomb survivors irradiated at Hiroshima and Nagasaki. Occupational exposure to ionising radiation has been incriminated in the first radiologists exposed to high doses. In nuclear industry workers, the results of epidemiological studies are contradictory and at the present time it is not possible to link their radiologic exposure with a risk of tumor of the nervous system. In populations living near nuclear plants, mortality due to tumors of the nervous system was not increased.