Bioterrorism preparedness plans must take into account the psychosocial consequences of exposure to mass violence. If possible, post-traumatic stress disorder (PTSD), which is associated with significant morbidity and cost, should be prevented. There are, however, no effective interventions that have been scaled up to prevent PTSD following mass exposure to violence. In fact, randomized controlled trials of the most commonly used preventive intervention, psychological debriefing, suggest no efficacy, or even potential harm. Fortunately, randomized controlled trials of cognitive behavioral therapy--that is, targeting individuals who are symptomatic in the weeks after trauma--reveal significant efficacy. Given the potential for repeated mass violence exposure, public health professionals need to refine methods for screening and tracking large numbers of casualties. At the same time, the use of telephone and internet-based cognitive behavioral therapy protocols should be further tested as strategies for bringing the only effective early intervention for PTSD to scale. Research on preventive pharmacotherapy for PTSD and on the effects of media exposure on PTSD severity is also a priority.