Stephen Joshua Sondheim (/ˈsɒndhaɪm/; March 22, 1930 – November 26, 2021) was an American composer and lyricist. Regarded as one of the most important figures in 20th-century musical theatre, he is credited with reinventing the American musical. With his frequent collaborators Harold Prince and James Lapine, Sondheim's Broadway musicals tackled unexpected themes beyond the genre's traditional subjects while addressing darker elements of the human experience. His music and lyrics are tinged with complexity, sophistication, and ambivalence about various aspects of life.

Sondheim's interest in musical theatre began at a young age, and Oscar Hammerstein II mentored him. He started his career by writing the lyrics for West Side Story (1957) and Gypsy (1959). He transitioned to writing both music and lyrics for the theatre, with his best-known works including A Funny Thing Happened on the Way to the Forum (1962), Company (1970), Follies (1971), A Little Night Music (1973), Sweeney Todd: The Demon Barber of Fleet Street (1979), Merrily We Roll Along (1981), Sunday in the Park with George (1984), and Into the Woods (1987).

Sondheim's numerous awards and nominations include eight Tony Awards, an Academy Award, eight Grammy Awards, an Olivier Award, and the Pulitzer Prize. He also was awarded the Kennedy Center Honor in 1993 and a Presidential Medal of Freedom in 2015. A theatre is named after him both on Broadway and in London's West End. Film adaptations of his works include West Side Story (1961), Gypsy (1962), A Funny Thing Happened on the Way to the Forum (1966), A Little Night Music (1977), Sweeney Todd: The Demon Barber of Fleet Street(2007), Into the Woods (2014), and West Side Story (2021).

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