The use of biological and chemical weapons as agents of warfare and terrorism has occurred sporadically, but recent events demonstrate the increasing risk and possibility that terrorist groups with grievances against the government or groups may employ them. Historically, most evaluations of the potential risk for biological weaponry have focused on the military, but the recent release of anthrax in the United States demonstrates that civilian populations are also at risk. More likely than not, most bioterrorism events will be of a small scale; however, agents such asBacillus anthracisandYersinia pestiscould leave hundreds of thousands dead or incapacitated. The impact of the attack will depend on a number of variables, including the agent used, method of dispersal, and the responsiveness of the public health system. With any large-scale event, the public health infrastructure will be called upon to deal with mass casualties and the “worried well.”