Various types of radiation accidents can occur, including rare events such as nuclear disasters, and mistakes in handling radiation sources. Although such accidents are less common in comparison to other disasters, a rapid and long-term medical response is necessary, especially in cases in which individuals are seriously injured by radiation exposure or contamination. In order to facilitate prompt medical treatment at the time of an emergency, it is necessary to understand the radiation dose to which the patient was exposed, to determine whether contamination occurred, and to select an appropriate treatment. In addition, we also need to have some basic knowledge of radiation to understand the associated dangers. In this article, we summarize the historical radiation accidents and report the details of treatment for patients who were exposed to radiation.