Stephen Joshua Sondheim was an American composer and lyricist. Regarded as one of the most important figures in 20th-century musical theater, he is credited with reinventing the American musical. With his frequent collaborators Harold Prince and James Lapine, Sondheim's Broadway musicals tackled unexpected themes that ranged beyond the genre's traditional subjects, while addressing darker elements of the human experience. His music and lyrics were tinged with complexity, sophistication, and ambivalence about various aspects of life.

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.