The Acid Tests were a series of parties held by author Ken Kesey primarily in the San Francisco Bay Area during the mid-1960s, centered on the use of and advocacy for the psychedelic drug LSD, commonly known as "acid". LSD was not made illegal in California until October 6, 1966.

Ken Elton Kesey was an American novelist, essayist and countercultural figure. He considered himself a link between the Beat Generation of the 1950s and the hippies of the 1960s.