Ionizing radiation injures tissues variably, depending on factors such as radiation dose, rate of exposure, type of radiation, and part of the body exposed. Symptoms may be local (eg, burns) or systemic (eg, acute radiation sickness). Diagnosis is by history of exposure, symptoms and signs, and sometimes use of radiation detection equipment to localize and identify radionuclide contamination. People exposed to radiation may be divided into ‘low-risk’ and ‘high-risk’ susceptibility groups, based on degree of neutropenia and the presence of comorbidities. Management focuses on associated traumatic injuries, decontamination, supportive measures, and minimizing exposure of health care workers. Patients with severe acute radiation sickness receive reverse isolation, antimicrobial and anti-inflammatory agents, and bone marrow support. Patients internally contaminated with certain specific radionuclides may receive uptake inhibitors or chelating agents. Prognosis is initially estimated by the time between exposure and symptom onset, the severity of those symptoms, and by the lymphocyte count during the initial 24 to 72 hours.