Chemical and biological weapons represent two distinct types of munitions that share some common policy implications. While chemical weapons and biological weapons are different in terms of their development, manufacture, use, and the methods necessary to defend against them, they are commonly united in matters of policy as “weapons of mass destruction,” along with nuclear and radiological weapons. Both chemical and biological weapons have the potential to cause mass casualties, require some technical expertise to produce, and can be employed effectively by both nation states and non-state actors. U.S. policies in the early 20th century were informed by preexisting taboos against poison weapons and the American Expeditionary Forces’ experiences during World War I. The United States promoted restrictions in the use of chemical and biological weapons through World War II, but increased research and development work at the outset of the Cold War. In response to domestic and international pressures during the Vietnam War, the United States drastically curtailed its chemical and biological weapons programs and began supporting international arms control efforts such as the Biological and Toxin Weapons Convention and the Chemical Weapons Convention. U.S. chemical and biological weapons policies significantly influence U.S. policies in the Middle East and the fight against terrorism.