Since the discovery of X-rays the practical use of ionizing radiation and its damaging effects have been a source of concern for occupational health and radiation protection. This led to the introduction of dose limits and strict controls associated with the use of radiation for civil uses. This AT lecture series will discuss the effects of radiation on humans with special emphasis on the health effects of low doses. Radiation risks as assessed from Hiroshima and Nagasaki, Chernobyl as well as others accidental and occupational exposures will be presented and discussed.