Grease is a 1971 musical with music, lyrics, and a book by Jim Jacobs and Warren Casey. Named after the 1950s United States working-class youth subculture known as greasers and set in 1959 at the fictional Rydell High School in Northwest Chicago (based on Taft High School in Chicago, Illinois, and named after rock singer Bobby Rydell), the musical follows ten working-class teenagers as they navigate the complexities of peer pressure, politics, personal core values, and love.

The score borrows heavily from the sounds of early rock and roll. In its original production in Chicago, Grease was a raunchy, raw, aggressive, vulgar show. Subsequent productions toned down the more risqué content. The show mentions social issues such as teenage pregnancy, peer pressure, and gang violence; its themes include love, friendship, teenage rebellion, sexual exploration during adolescence. Jacobs described the show's basic plot as a subversion of common tropes of 1950s cinema, since the female lead, who in many 1950s films transformed the alpha male into a more sensitive and sympathetic character, is instead drawn into the man's influence and transforms into his wild, roguish fantasy.

Since it was first performed on February 5, 1971, at Kingston Mines nightclub in Chicago, Grease has been successful on both stage and screen, but the content has been diluted and its teenage characters have become less Chicago habitués (the characters' Polish-American backgrounds in particular are ignored with last names often changed, although two Italian-American characters are left identifiably ethnic) and more generic. The first Broadway production opened on June 7, 1972; when it closed in 1980, Grease's 3,388-performance run was the longest yet in Broadway history, although it was surpassed by A Chorus Line on September 29, 1983. It went on to become a West End hit, a successful feature film, two popular Broadway revivals in 1994 and 2007, and a staple of regional theatre, summer stock, community theatre, and high school and middle school drama groups. It remains Broadway's 18th longest-running show.

Grease was adapted in 1978 as a feature film, which starred John Travolta (who himself had been in stage productions in a different role) and British-Australian singer and actress Olivia Newton-John, removed the musical's Chicago urban setting, and changed some plot elements, characters, and songs while adding new songs and elaborating on some plot elements only alluded to in the musical. Some of these revisions have been incorporated into revivals of the musical. A 2016 live TV musical used elements from both the original stage version and the film. A 1982 film sequel, Grease 2, included only a few supporting characters from the film and musical and had no involvement from Jacobs or Casey; Jacobs has gone on record to voice his disapproval of Grease 2.

The show's original production was directed by Guy Barile, choreographed by Ronna Kaye and produced by the Kingston Mines Theatre Company founded by June Pyskacek on Chicago's Lincoln Avenue. The script was based on Jim Jacobs' experience at William Taft High School, Chicago. Warren Casey collaborated with Jim and together they wrote the music and lyrics. It ran for eight months. The cast: Doug Stevenson (Danny), Leslie Goto (Sandy), Sue Williams (Rizzo), Polly Pen (Patty), Gary Houston (Roger), Marilu Henner (Marty), James Canning (Doody), Hedda Lubin (Frenchy), Bruce Hickey (Kenickie), Sheila Ray Ceaser (Jan), Bill Cervetti (Miller), Jerry Bolnick (Sonny), Judy Brubaker (Miss Lynch), Mike O'Connor (Vince Fontaine), Steve Munro (Eugene), Barbara Munro (Cha Cha), Mac Hamilton (Teen Angel) and George Lopez (Bum). In addition to the "R-rated" profanity and deliberate use of shock value, the Chicago version of Grease included a substantially different songbook, which was shorter and included multiple references to real Chicago landmarks.

Producers Ken Waissman and Maxine Fox saw the show and made a deal to produce it Off-Broadway. The team headed to New York City to collaborate on the New York production of Grease. It was after test runs of the original Chicago production had drawn extremely poor reviews that the production team transformed Grease into its familiar form. The new production, directed by Tom Moore and choreographed by Patricia Birch (who later choreographed the film adaptation, and directed the ill-fated sequel), opened Off-Broadway at the Eden Theatre in lower Manhattan on February 14, 1972. Though Grease opened geographically off-Broadway, it did so under first-class Broadway contracts. The show was deemed eligible for the 1972 Tony Awards, receiving seven Tony Award nominations.

On June 7, 1972, the production moved to the Broadhurst Theatre on Broadway, and on November 21, it moved to the Royale Theatre there, where it ran until January 27, 1980. For the five final weeks of the run, the show moved to the larger Majestic Theatre. By the time it closed on April 13, 1980, it had run 3,388 performances.

The original Broadway cast included Barry Bostwick as Danny, Carole Demas as Sandy, Adrienne Barbeau as Rizzo, and Timothy Meyers as Kenickie, with Alan Paul, Walter Bobbie and Marya Small in supporting roles. Replacements later in the run included Jeff Conaway as Danny, Candice Earley as Sandy, John Lansing as Danny, Peter Gallagher as Danny, Richard Gere as Sonny, Ilene Graff as Sandy, Randee Heller as Rizzo, Marilu Henner as Marty, Judy Kaye as Rizzo, Marcia Mitzman Gaven as Rizzo, Patrick Swayze as Danny, John Travolta as Doody, Treat Williams as Danny, Laurie Graff as Frenchy, and Jerry Zaks as Kenickie.