Stephen Arthur Stills (born January 3, 1945) is an American musician, singer, and songwriter best known for his work with Buffalo Springfield, Crosby, Stills & Nash, and Manassas. As both a solo act and member of three successful bands, Stills has combined record sales of over 35 million albums. He was ranked number 28 in Rolling Stone's 2003 list of "The 100 Greatest Guitarists of All Time" and number 47 in the 2011 list. Stills became the first person to be inducted twice on the same night into the Rock and Roll Hall of Fame. According to Neil Young, "Stephen is a genius".

Beginning his professional career with Buffalo Springfield, he composed "For What It's Worth," which became one of the most recognizable songs of the 1960s. Other notable songs he contributed to the band were "Sit Down, I Think I Love You", "Bluebird", and "Rock & Roll Woman". According to bandmate Richie Furay, Stills was "the heart and soul of Buffalo Springfield".

After Buffalo Springfield disbanded, Stills began working with David Crosby and Graham Nash as the trio called Crosby, Stills & Nash (CSN). In addition to writing many of the band's songs, Stills played bass, guitar, and keyboards on their debut album. The album sold over four million copies and at that point had outsold anything from the three members' prior bands: the Byrds, Buffalo Springfield, and the Hollies. The album won the trio a Grammy Award for Best New Artist.

Stills' first solo album, Stephen Stills, earned a gold record and is the only album to feature both Jimi Hendrix and Eric Clapton. Its hit single "Love the One You're With" became his biggest solo hit, peaking at number 14 on the Billboard Hot 100. Stills followed this with a string of solo albums, as well as starting a band with Chris Hillman called Manassas in 1972. In summer 1974, Young reunited with CSN after a four-year hiatus for a concert tour that was recorded and released in 2014 as CSNY 1974. It was one of the first stadium tours and the largest tour the band has done to date. CSN reunited in 1977 for their album CSN, which became the trio's best-selling record. CSN and CSNY continued to have platinum albums through the 1980s.