The United States (U.S.) must prepare for a wide variety of low probability attacks on its turf, as well as plan its defense policies and programs in which there is no way to predict the weapon that will be used. The risks posed by chemical, biological, nuclear, and radiological weapons differ sharply in character and in their effects. Each form of weapon can be used in ways that present radically different problems in defense and response. Biological weapons employ living agents or toxins produced by natural or synthetic agents to kill or injure humans, domestic animals, and crops. They are also nearly ideal terror weapons with massive psychological as well as physiological consequences. Such weapons fall into five main medical categories: bacterial agents such as anthrax, plague, typhoid fever; rickettsial agents such as typhus and Rocky Mountain spotted fever; viral agents such as smallpox, yellow fever, Ebola; toxins such as botulinum and aflatoxin; and fungal such as coccidioidomycosis. Biological weapons can be a weapon of mass destruction with which most first responders and law enforcement agencies are able to deal. Most responders feel that they already have to prepare for such incidents, and the estimated total casualties are unlikely to put an impossible burden on local and regional medical services. Law enforcement experts believe most incidents will have a clear location and clear chains of evidence. However, the threat of biological weapons illustrates the need to be able to measure the existing capabilities of Federal, State, and local defenders and responders; to determine what can be done to improve their capabilities with minimal or no additional resources; and then to address what level of additional capability the Nation is and is not willing to fund. 10 tables and 88 notes.