Although the threat of chemical and biological warfare has grown, the actual use of poisons as weapons of war or terrorism remains rare. The reason rests in part on a long-standing taboo about poison weapons. This article explores possible biological and cultural explanations for the taboo; surveys the use of poisons by other species, by tribal groups, and by ancient societies; and considers the importance of snakes in developing attitudes about poisons. Reluctance to use poison weapons may have had its origin in the linkage of medicine, poisons, and mysticism common in societies everywhere. Whatever the reasons, however, antipathy to poison weapons is deep-seated, and most nations have embraced the norm that deems these weapons morally repugnant. Policies should be sought that enhance this attitude and therefore make the use of such weapons less likely.