The expected number of people infected by an atmospheric release of a biological agent depends on the physical and pathogenic properties of the agent, the amount of agent released, the mechanism by which it is dispersed, atmospheric transport processes, environmental degradation of the agent, and the protection afforded by being inside a building, for those who happen to be inside of buildings, when the plume passes by outdoors. Using anthrax as a test case, this research examined each of these factors in detail, determining nominal values for representative parameters and, more importantly, assessed the range of uncertainty or the lack of scientific knowledge regarding these parameters. The dominant factors affecting the outcome of hypothetical bioterror attacks are the weather, the precise urban area in which the release occurs, the exact form of the dose-response relationship for inhalation anthrax in humans, and the magnitude of the source term.