The terrorist attacks on the United States in 2001 and the anthrax release soon after brought the issue of bioterrorism to the forefront in the medical community. Bioterrorism is the use of a biologic weapon to create terror and panic. Biologic weapons, or bioweapons, can be bacteria, fungi, viruses, or biologic toxins. Because the emergency department represents the front line of defense for the recognition of agents of bioterrorism, it is essential that emergency physicians have the ability to quickly diagnose victims of bioterrorism. This review examines the most deadly and virulent category A agents of bioterrorism, that is, anthrax, smallpox, plague, botulism, hemorrhagic fever viruses, and tularemia. The focus is on epidemiology, transmission, clinical manifestations, diagnosis, and treatment.