After radiation accidents, studies on survivors and animal experiments have revealed that radiation exposure has harmful chronic and acute effects. The duration of these effects is proportional to the radiation dose received. These effects are analyzed in two classes as chronic and acute effects. Acute effects, which are the subject of this chapter, occur within minutes, hours and days. Acute radiation syndrome usually occurs in regional and whole body irradiation at doses of 1 Gy and above. Today, data on acute radiation syndrome are usually obtained as a result of radiation accidents. Acute radiation syndrome is analyzed in three groups as Hematopoietic Syndrome, Gastrointestinal Syndrome and Cerebrovascular Syndrome in relation to the radiation dose received. In total body irradiation, dose exposure is controlled and fractionated. For this reason, it is unlikely to observe severe conditions similar to acute radiation syndrome that may occur as a result of a radiation accident. However, it is possible to observe similar clinical pictures.  In this section, the clinical significance of acute radiation syndrome as a result of a radiation accident will be analyzed and its clinical significance in whole body irradiation will be evaluated.