Accidents and disasters involving ionizing radiation are rare. Such accidents may occur not only at nuclear power stations but also in medicine and in industry. Only fairly large radiation doses can give acute medical effects. Symptoms and signs depend on the actual dose, and on what parts of the body are irradiated. In connection with a radiation accident, the main points are to recognize in the first place that radiation may be involved, and to have some knowledge of the actual source. Risk of localized irradiation from industrial sources, especially of hands, is a problem that is often overlooked. The paper reviews the principles for action by local health officers, the pathogenesis of radiation injury and early medical management. Some information is also given on the Norwegian system of contingency preparedness against nuclear accidents.