On 22--23 June 2001, the Johns Hopkins Center for Civilian Biodefense Strategies, in collaboration with the Center for Strategic and International Studies, the Analytic Services Institute for Homeland Security, and the Oklahoma National Memorial Institute for the Prevention of Terrorism, held a senior-level exercise entitled "Dark Winter" that simulated a covert smallpox attack on the United States. The first such exercise of its kind, Dark Winter was constructed to examine the challenges that senior-level policy makers would face if confronted with a bioterrorist attack that initiated outbreaks of highly contagious disease. The exercise was intended to increase awareness of the scope and character of the threat posed by biological weapons among senior national security experts and to bring about actions that would improve prevention and response strategies.