Since the development of x-rays in the late 19th century, radiation has increasingly been used for diagnostic imaging and various industrial purposes. The human health consequences of radiation exposure can range from minor asymptomatic exposures to the development of potentially fatal acute radiation sickness depending on the characteristics of the exposure. This review provides an overview of radiation injury, including historical perspective and principles of toxicity; assessment and stabilization (bedside evaluation, supportive care, and empirical therapy); diagnosis (estimation of radiation dose, laboratory assessment, biodosimetry and bioassays, smoke inhalation, and psychiatric sequelae); treatment and disposition (hematologic acute radiation illness, countermeasures and antidotes, gastrointestinal symptoms, fluid and electrolyte management, surgical intervention, local radiation injury, psychiatric treatment, radiation exposure during pregnancy, palliative care, and disposition); and outcomes (carcinogenesis, noncancer effects). Tables include the characteristic of common radioactive isotopes, common radiation dose equations and conversions, prescribing information for colony-stimulating factors, recommended antimicrobial prophylaxis and treatment regimens, and radioisotope decorporation or blocking agents. Figures show types of ionizing radiation, comparison of radiation contamination and exposure, patterns of early lymphocyte response based on radiation dose, the relationship between time to onset of vomiting and radiation dose, and a sample body chart for documentation of radiation contamination. This review contains 5 figures, 5 tables, and 82 references. Key Words: radiation, radiation exposure, radiation sickness, diagnostic imaging, pregnancy, carcinogenesis, toxicity, acute radiation syndrome (ARS)