In radiological scenarios, many individuals may be subjected to ionizing radiation at low doses or low dose rates. These exposures may result from fallout fields, radiological dispersal devices (“dirty bombs”), nuclear accidents or space travel. The primary concern from such exposures is not Acute Radiation Syndrome (ARS), but late effects such as cancer. The molecular mechanisms leading to these effects are different from those causing acute effects, and the mechanisms are being investigated at AFRRI. A better understanding of low dose and dose rate effects will lead to improved radiation protection guidance and will suggest targets for pharmacological countermeasures. AFRRI has been investigating countermeasure candidates that prevent radiation-induced leukemia.