Hair: The American Tribal Love-Rock Musical is a rock musical with a book and lyrics by Gerome Ragni and James Rado and music by Galt MacDermot. The work reflects the creators' observations of the hippie counterculture and sexual revolution of the late 1960s, and several of its songs became anthems of the anti-Vietnam War peace movement. The musical's profanity, its depiction of the use of illegal drugs, its treatment of sexuality, its irreverence for the American flag, and its nude scene caused much comment and controversy. The work broke new ground in musical theatre by defining the genre of "rock musical", using a racially integrated cast, and inviting the audience onstage for a "Be-In" finale.Hair tells the story of the "tribe", a group of politically active, long-haired hippies of the "Age of Aquarius" living a bohemian life in New York City and fighting against conscription into the Vietnam War. Claude, his good friend Berger, their roommate Sheila and their friends struggle to balance their young lives, loves, and the sexual revolution with their rebellion against the war and their conservative parents and society. Ultimately, Claude must decide whether to resist the draft as his friends have done, or to serve in Vietnam, compromising his pacifist principles and risking his life.

After an off-Broadway debut on October 17, 1967, at Joseph Papp's Public Theater and a run at the Cheetah nightclub from December 1967 through January 1968, the show opened on Broadway in April 1968 and ran for 1,750 performances. Simultaneous productions in cities across the United States and Europe followed shortly thereafter, including a successful London production that ran for 1,997 performances. Since then, numerous productions have been staged around the world, spawning dozens of recordings of the musical, including the 3 million-selling original Broadway cast recording. Some of the songs from its score became Top 10 hits, and a feature film adaptation was released in 1979. A Broadway revival opened in 2009, earning strong reviews and winning the Tony Award and Drama Desk Award for Best Revival of a Musical. In 2008, Time wrote, "Today Hair seems, if anything, more daring than ever."

Musical theatre is a form of theatrical performance that combines songs, spoken dialogue, acting and dance. The story and emotional content of a musical – humor, pathos, love, anger – are communicated through words, music, movement and technical aspects of the entertainment as an integrated whole. Although musical theatre overlaps with other theatrical forms like opera and dance, it may be distinguished by the equal importance given to the music as compared with the dialogue, movement and other elements. Since the early 20th century, musical theatre stage works have generally been called, simply, musicals.

Although music has been a part of dramatic presentations since ancient times, modern Western musical theatre emerged during the 19th century, with many structural elements established by the works of Gilbert and Sullivan in Britain and those of Harrigan and Hart in America. These were followed by the numerous Edwardian musical comedies and the musical theatre works of American creators like George M. Cohan at the turn of the 20th century. The Princess Theatre musicals (1915–1918) were artistic steps forward beyond revues and other frothy entertainments of the early 20th century and led to such groundbreaking works as Show Boat (1927), Of Thee I Sing (1931) and Oklahoma! (1943). Some of the most famous musicals through the decades that followed include

My Fair Lady (1956), The Fantasticks (1960), Hair (1967), A Chorus Line (1975), Les Misérables (1985), The Phantom of the Opera (1986), Rent (1996), The Producers (2001), Wicked (2003) and Hamilton (2015). In 2020–2021, many musical theatre productions were shut down due to the Covid-19 pandemic.

Musicals are performed around the world. They may be presented in large venues, such as big-budget Broadway or West End productions in New York City or London. Alternatively, musicals may be staged in smaller venues, such as fringe theatre, off-Broadway, off-off-Broadway, regional theatre, or community theatre productions, or on tour. Musicals are often presented by amateur and school groups in churches, schools and other performance spaces. In addition to the United States and Britain, there are vibrant musical theatre scenes in continental Europe, Asia, Australasia, Canada and Latin America.