Nuclear terrorism has been an essential part of national preparedness since the formation of the U.S. Department of Homeland Security (DHS),1 but until recently little research had been done on the potential effects and mitigation strategies specific to a low-yield, ground-level nuclear detonation in a modern U.S. city. An effective response will involve large-scale measures, mass casualty management, mass evacuations, and mass decontamination. Preparedness planning for this scenario presents especially difficult challenges in time-critical decision making and the management of a large number of casualties in the hazard area. Perhaps even more challenging will be coordinating a large-scale response that involves multiple jurisdictions with limited infrastructure and limited resources.In 2007, Congress, concerned that cities had little guidance to help them prepare their populations to react in the critical minutes after a nuclear terrorism event, directed the DHS Office of Health Affairs (OHA) to work with the National Academies Institute of Medicine, the Homeland Security Institute, the national laboratories, and state and local response organizations to address this issue (U.S. Congress, 2007). The OHA initiative is currently managed by the Federal Emergency Management Agency (FEMA) as part of a coordinated federal effort to improve response planning for a nuclear detonation.At the start of OHA’s efforts, there appeared to be no scientific consensus on actions that should be taken after a nuclear detonation. For example, the recommendations on the DHS website, Ready.gov, are consistent with the recommendations of the National Academies (2005) but were criticized by the Federation of American Scientists (2006) based on conflicting recommendations in a RAND study (Davis et al., 2003; Orient, 2005).