The intentional dispersal of biological agents by terrorists is a potential problem that increasingly concerns the intelligence, law enforcement, medical, and public health communities. Terrorists might choose biological agents over conventional and chemical weapons for multiple reasons, although it is difficult to predict, with certainty, which biological agents might prove attractive to terrorists. One can more confidently, however, derive a list of those few agents which, if used, would be of greatest public health consequence. It is these agents which will require the most robust countermeasures. We discuss the derivation of this short list of agents and the specific diseases involved.