Ian Ernest Gilmore Evans (born Green; May 13, 1912 – March 20, 1988) was a Canadian-American jazz pianist, arranger, composer and bandleader. He is widely recognized as one of the greatest orchestrators in jazz, playing an important role in the development of cool jazz, modal jazz, free jazz, and jazz fusion. He is best known for his acclaimed collaborations with Miles Davis.

Gil Evans was born to Ian Ernest Green and Margaret Julia McConnachy on May 13, 1912. Originally named Ian Ernest Gilmore Green, he would eventually change his name from Green to Evans, taking the name of his step-father, John Evans. His father was a doctor and his mother was a homemaker and he had two siblings, Jean and Montgomery.

From a young age, Evans moved many times; from his birthplace in Toronto to western Canada, then to Washington State before finally settling in Stockton, California. He graduated from Stockton High School and Modesto Junior College. Evans remained a Canadian citizen until he entered the US Army during the second World War. After 1946, he lived and worked primarily in New York City, living for many years at Westbeth Artists Community.

Between 1941 and 1948, Evans worked as an arranger for the Claude Thornhill Orchestra. Even then, early in his career, his arrangements were such a challenge to musicians that bassist Bill Crow recalled that bandleader Thornhill would bring out Evans’s arrangements “when he wanted to punish the band.” Evans’ modest basement apartment behind a New York City Chinese laundry soon became a meeting place for musicians looking to develop new musical styles outside of the dominant bebop style of the day. Those present included the leading bebop performer, Charlie Parker, as well as Gerry Mulligan and John Carisi. In 1948, Evans, with Miles Davis, Mulligan, and others, collaborated on a band book for a nonet. These ensembles, larger than the trio-to-quintet “combos”, but smaller than the “big bands” which were on the brink of economic unviability, allowed arrangers to have a larger palette of colors by using French horns and tuba. Claude Thornhill had employed hornist John Graas in 1942, and composer-arranger Bob Graettinger had scored for horns and tubas with the Stan Kenton orchestra, but the “Kenton sound” was in the context of a dense orchestral wall of sound that Evans avoided.