Agent Orange is a chemical herbicide and defoliant, one of the "tactical use" Rainbow Herbicides. It was used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. It is a mixture of equal parts of two herbicides, 2,4,5-T and 2,4-D. In addition to its damaging environmental effects, traces of dioxin (mainly TCDD, the most toxic of its type) found in the mixture have caused major health problems for many individuals who were exposed, and their offspring.

Agent Orange was produced in the United States from the late 1940s and was used in industrial agriculture, and was also sprayed along railroads and power lines to control undergrowth in forests. During the Vietnam War, the U.S. military procured over 20 million gallons, consisting of a fifty-fifty mixture of 2,4-D and dioxin-contaminated 2,4,5-T. Nine chemical companies produced it: Dow Chemical Company, Monsanto Company, Diamond Shamrock Corporation, Hercules Inc., Thompson Hayward Chemical Co., United States Rubber Company (Uniroyal), Thompson Chemical Co., Hoffman-Taff Chemicals, Inc., and Agriselect.

The government of Vietnam says that up to four million people in Vietnam were exposed to the defoliant, and as many as three million people have suffered illness because of Agent Orange, while the Vietnamese Red Cross estimates that up to one million people were disabled or have health problems as a result of exposure to Agent Orange. The United States government has described these figures as unreliable, while documenting cases of leukemia, Hodgkin's lymphoma, and various kinds of cancer in exposed U.S. military veterans. An epidemiological study done by the Centers for Disease Control and Prevention showed that there was an increase in the rate of birth defects of the children of military personnel as a result of Agent Orange. Agent Orange has also caused enormous environmental damage in Vietnam. Over 3,100,000 hectares (31,000 km² or 11,969 mi²) of forest were defoliated. Defoliants eroded tree cover and seedling forest stock, making reforestation difficult in numerous areas. Animal species diversity is sharply reduced in contrast with unsprayed areas.

The use of Agent Orange in Vietnam resulted in numerous legal actions. The United Nations ratified United Nations General Assembly Resolution 31/72 and the Environmental Modification Convention. Lawsuits filed on behalf of both U.S. and Vietnamese veterans sought compensation for damages.

Agent Orange was first used by the British Armed Forces in Malaya during the Malayan Emergency. It was also used by the U.S. military in Laos and Cambodia during the Vietnam War because forests near the border with Vietnam were used by the Viet Cong.