Ian Ernest Gilmore Evans (né 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 in Toronto, Canada, on May 13, 1912, to Margaret Julia McConnachy. Little is known about Evans's biological father, although a family friend said that he was a doctor who died before Evans was born. Originally named Gilmore Ian Ernest Green, Evans took the last name of his step-father, John Evans, a miner. The family moved frequently, living in Saskatchewan, British Columbia, Washington, Idaho, Montana, and Oregon, migrating to wherever Evans's father could find work. Eventually, the family ended up in California, first in Berkeley, where Evans attended the ninth and tenth grades, then in Stockton, where he attended Stockton High School, graduating in 1930. During this time, Evans's father took a job with the Western Pacific Railroad, and Evans began living with friends, later renting a room in a boarding house. After graduating, Evans attended the College of the Pacific in Stockton, where he founded his first jazz big band, Gil Evans & His Orchestra, in 1933, but then transferred to Modesto Junior College. After a year in Modesto, Evans left and moved back to Stockton.

Evans became interested in music at an early age, listening to Duke Ellington, Louis Armstrong, and Fletcher Henderson on the radio and on records. He studied the piano and began learning how to arrange music, and started picking up jobs with local musicians. While in college, he founded his first band, which performed his arrangements, and which became the house band at the Rendezvous Ballroom in Balboa Beach, California, in 1935. The band toured the Pacific Northwest in 1937 and eventually settled in Hollywood, where they regularly performed on Bob Hope's radio show. Evans's arrangements from this time showed the influence of classical music, and included instruments such as French horns, flutes, and tubas. In 1939, Claude Thornhill was hired for Hope's show, and he became a major influence on Evans.

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's 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 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. The Miles Davis-led group was booked for a week at the Royal Roost as an intermission group on the bill with the Count Basie Orchestra. Capitol Records recorded 12 numbers by the nonet at three sessions in 1949 and 1950. These recordings were reissued on a 1957 Miles Davis LP titled Birth of the Cool.

Later, while Davis was under contract with Columbia Records, producer George Avakian suggested that Davis could work with any of several arrangers. Davis immediately chose Evans. The three albums that resulted from the collaboration are Miles Ahead (1957), Porgy and Bess (1958), and Sketches of Spain (1960). Another collaboration from this period, Quiet Nights (1962) was issued later, against the wishes of Davis, who broke with his then-producer Teo Macero for a time as a result. Although these four records were marketed primarily under Davis's name (and credited to Miles Davis with Orchestra Under the Direction of Gil Evans), Evans's contribution was as important as Davis's. Their work coupled Evans's classic big band jazz stylings and arrangements with Davis's solo playing. Evans also contributed behind the scenes to Davis's classic quintet albums of the 1960s.

The demands of the score for Porgy and Bess were legendary, from the very first note for the lead trumpet. The limited time allotted for rehearsals revealed that the ability to read such a challenging score was not consistent among jazz musicians, and there are many audible errors. Yet the recording is now regarded by many as one of the greatest reinterpretations of Gershwin's music in any musical style, because Evans and Davis were each devoted to going outside the "mainstream" of commercial expectations for jazz musicians. Evans was a great influence on Davis's interest in "non-jazz" music, especially orchestral music. Unfortunately, Evans's orchestral scores from the Porgy and Bess sessions were later found to be incomplete (or simply lost), and Quincy Jones and Gil Goldstein attempted to reconstruct these for Miles Davis's final 1991 concerts at Montreux, recorded as Miles & Quincy Live at Montreux. Davis had relented after years of refusing to revisit this material, but he was clearly ill, recovering from pneumonia, and trumpeter Wallace Roney, who was mentored by Davis, covered many of the challenging passages. Davis died before the release of the album.