"You Light Up My Life" is a ballad written by Joseph Brooks, and originally recorded by Kasey Cisyk for the soundtrack album to the 1977 film of the same title. The song was lip synced in the film by its lead actress, Didi Conn. The best-known cover version of the song is a cover by Debby Boone, the daughter of singer Pat Boone. It held the No. 1 position on the Billboard Hot 100 chart for ten consecutive weeks in 1977 and topped Record World magazine's Top 100 Singles Chart for a record 13 weeks.

Cisyk's original soundtrack recording was included in the film's soundtrack album. It was then later released as a single to bolster sales of the soundtrack album after Debby Boone included her version on her first solo album (also titled You Light Up My Life). Although the soundtrack album was certified Gold, peaking at No. 17 on the Billboard 200 albums chart, it never included Boone's hit single version of the song.

Cisyk's single was credited to "Original Cast", not to Cisyk herself, and even though Brooks is listed on the A-side of the single, the "Original Cast" B-side charted on the Billboard Hot 100 and only reached No. 80. Brooks also released an instrumental version of the song from the soundtrack as a promotional single, but that version failed to chart.

Following the success of Boone's version, the song earned Brooks a Grammy Award for Song of the Year, an Academy Award for Best Original Song, a Golden Globe Award for Best Original Song and an American Society of Composers, Authors and Publishers (ASCAP) award.

7" vinyl single

In a 2013 biographical essay about Cisyk, Cisyk's second husband, Ed Rakowicz (who worked as a sound engineer, but not for this song), wrote that songwriter Brooks was initially pleased with Cisyk's recording of the song with orchestra (and her version appeared in the movie and soundtrack) but "tried to evade payment by false promises and by asking her to be an incidental actor in his film, implying huge rewards yet to come..." Rackowicz claimed that Brooks made improper advances toward Cisyk, that after being rebuffed, he refused to speak directly to her again, and that he continued to evade payments to her while commissioning another recording with Debby Boone.

According to Rackowicz, "Besides wanting Boone to copy Kacey's [sic] iconic hit reading of his songs, Brooks needed to cover up Kacey's vocal leakage in the microphones in the piano recorded at the original demo session on which was overdubbed the orchestral track used in the film. Brooks didn't want to pay to re-record the piano and orchestra again." In 2003, Boone admitted, "I had no freedom whatsoever. Joe told me exactly how to sing it and imitate every inflection from the original recording." in an interview with Entertainment Weekly Magazine. Cisyk later retained a lawyer and sued Brooks for the fees she had earned for her work on the record and for credit on the soundtrack, which she later received.

In 1977, Debby Boone, Pat Boone's daughter, recorded the song under the guidance of Curb Records executive Mike Curb and songwriter Joseph Brooks. Boone recorded her vocals over a pre-existing instrumental track that Brooks already had developed for the film's soundtrack. The song was released as both a Warner-Curb Records single and as the title track to her first solo album, You Light Up My Life, which she released on Warner Bros. Records, Curb Records' parent label.