here has been concern with the risks of ionizing radiation since the beginning of the 20th century. Research on the subject has been increased in parallel with the rapid development of the medical uses of ionizing radiation. Also the first victims of radiation were the phycisists and doctors involved in research in radiology. This helps to explain why for nearly a century the effects of ionizing radiation on man have been studies more intensely than those of other noxious agfents, physical or chemical. The feeling of guilt which Hiroshima and Nagasaki produced in the scientific community explains why the study of radiopahology became eveen more extensive after 1945. Several hundred thousand people in hospitals and clinics work with ionizing radiation (diaganostic, nuclear medicine, radiotherapy). A million patients are treated each year by radiotherapy and many millions are examined by diagnostic radiology. About a hundred thousand people work in nuclear establishments. In spite of the large number of people exposed, accidental irradiation are rare in hospitals and very exceptional in nuclaer establishment. In the whole world the number of deaths due to accident irradiation was less than 30 during the 40 years up to the date of accident at Chernobyl. These remarkable results are due to a rigorous organization of radiation protection, which specialists in hygiene consider as a model for protection against noxious agents. Knowledge of the biological effects of ionzing radiation in man is based on the study of several hundred thousand patients treated with radiation, several groups of workers (painters of illuminous dails, radiologists), and the survivors at Hiroshima and Nagasaki. In addition of early somatic effects by exposure above the threhold of tolerance, there are two types of late effect. One is caused by the death of a large number of cells in a tissue (somatic effect) or a embry (teratogenic effect). The other type, referred to as stochastic, originates from a defect ocurring in a single cell : carcinogesis and genetic effects.