These viruses can be manipulated easily in the laboratory, and they have long served as model systems by which to study various aspects of virus replication, pathogenesis, induction of immune responses, and virus–-vector relation-ships. As a result, the alphaviruses are well described and their characteristics well defined. Therefore, the collective in vitro and in vivo characteristics of alphaviruses, especially the equine encephalomyelitis viruses, lend them-selves very well to weaponization. This fact was recognized by the designers of offensive biological warfare programs that were initiated before or during World War II. Few virus possess as many of the required characteristics for strategic or tactical weapons development as the alphaviruses.