Biological weapons use plants, animals, or substances derived from them to cause death or inflict harmful effects on the health of humans, animals, or plants that come in contact with the agents. Biological agents are defined as “pathogenic microorganisms and infective materials derived from them” (Stockholm International Peace Research Institute, 1973, vol. III, p.13). Definitions, however, can be problematic, especially as modern science blurs the distinction between toxins from naturally occurring plant and fungal tissues and the same toxins developed chemically. Whether toxins are designed for offensive or defensive use also blurs the issue. This entry begins with a discussion of the origins of biological weapons in conflicts before the 20th century and goes on to describe the development of biological weapons and their use ...