Biological weapons have been used in war from the start of recorded history. This article reviews the history of the subject, including the outbreak of the Black Death and the use of smallpox against American Indians. The new science of microbiology was misused from soon after its start and, despite the 1925 Geneva Protocol, the Japanese experimented extensively on prisoners in China. The Allies carried out extensive research during the Second World War, notably the United Kingdom into anthrax on Gruinard Island and the United States into a variety of agents. Despite the 1972 Biological Weapons Convention (BWC), a major programme continued in the former Soviet Union (leading to an accidental outbreak of anthrax). Most recently Iraq was revealed as having an extensive programme, with weaponization of large amounts of various agents, and several terrorists groups have attempted to use biological agents as weapons. Modern developments in biotechnology could lead to even more serious developments, and effective preventive measures, including strengthening of the BWC, are imperative.