One of the greatest national security threats to the United States is the detonation of an improvised nuclear device or a radiological dispersal device in a heavily populated area. The US Government has addressed these threats with a two-pronged strategy of preventing organizations from obtaining weapons of mass destruction and preparing in case an event occurs. The National Institute of Allergy and Infectious Diseases (NIAID) contributes to these preparedness efforts by supporting basic research and development for chemical, biological, radiological, and nuclear countermeasures for civilian use. The Radiation Countermeasures Program at NIAID has established a broad research agenda focused on the development of new medical products to mitigate and treat acute and long-term radiation injury, promote the clearance of internalized radionuclides, and facilitate accurate individual dose and exposure assessment. This paper reviews the recent work and collaborations supported by the Radiation Countermeasures Program.