Agent Orange is the code name for a powerful herbicide and defoliant used widely by the U.S. military during the Vietnam War. Agent Orange was used from 1961 to 1971 and has disputedly caused serious harm to the health of exposed Vietnamese, Canadians and Americans, their children and grandchildren.

Agent Orange is a roughly 1:1 mixture of the herbicides 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid). These herbicides were developed during the 1940s for use in controlling broad-leaf plants. First introduced in 1947, both of these herbicides had widespread use in agriculture by the middle of the 1950s.

During the Vietnam War, Agent Orange's official military purpose was to remove the leaves of trees to prevent guerrilla fighters of the National Liberation Front from hiding. Agent Orange is a colorless liquid: its name was from the color of the stripes on the barrels used to transport it.

Agent Orange as a military defoliant was discontinued in 1971, after over 6,000 spraying missions in Vietnam and Cambodia; 2,4-D continues to be widely used as an herbicide. The use of 2,4,5-T has been banned in the U.S. and many other countries.