"I Know Him So Well" is a duet from the concept album and subsequent musical Chess by Tim Rice, Benny Andersson and Björn Ulvaeus. It was originally sung by Elaine Paige (as Florence) and Barbara Dickson (as Svetlana). In this duet, two women – Svetlana, the Russian chess champion's estranged wife, and Florence, his mistress – express their bittersweet feelings for him and at seeing their relationships fall apart.

The chorus of the song is based on the chorus of "I Am an A", a song performed live during Andersson and Ulvaeus' group ABBA's 1977 tour.^[2] Although "I Am an A" was never released officially, it circulated on various bootlegs and is readily available on YouTube.

The duet was first released worldwide on the Chess double LP, often referred to as a concept album or album musical, in the autumn of 1984. Later it was released as a single by Paige and Dickson, the duet reaching number one in the UK Singles Chart for four weeks in 1985.^[3] They laid down their vocals separately and never met during the recording of the song, only for the video and subsequent performances on Top of the Pops and the European tours.^[1][4] This recording remains in the Guinness Book of Records as the biggest selling UK chart single ever by a female duo. Paige also appeared in the original London West End stage production of Chess. The song peaked at number 21 in Australia.^[5]

In the United Kingdom on 18 September 2004, the BBC's All-Time Greatest Love Songs (hosted by Lionel Richie) saw the duet performed live by Paige and Dickson, together for the first time in 20 years, to a rapturous reception. Paige played her solo demo recording of the song on her BBC Radio 2 show, Elaine Paige on Sunday.

In January 2011, the Official Charts Company released a list of the top 10 All Time Best Selling Duets in which "I Know Him So Well" was placed seventh in the chart.^[6]

Paige and Dickson also recorded Spanish-language lyrics for release as "Lo Creo Conocer". The single was never released though.^[7]

In 1987, a studio version of "I Know Him So Well" was recorded as a duet by Cissy Houston and her daughter Whitney Houston for Whitney's second album, Whitney. It was released as the 6th and final single from Whitney on 30 November 1988 in Australia, Germany, Netherlands, and Spain. There was no official music video produced for the Houston duet of song, however there are live performance video-recordings. The song version was included in the North American leg of Houston's Moment of Truth World Tour. The single was not promoted as heavily as previous singles from Whitney.

British musical group Steps originally recorded "I Know Him So Well" for the 1999 Abbamania compilation, and the song later appeared on Steps' own The Last Dance collection. It was released as a single together with "Words Are Not Enough" in 2001 as the group's fifteenth single release, and the group's last single to be released before their Boxing Day split later the same month.

"I Know Him So Well" was recorded by Peter Kay and Susan Boyle for Comic Relief 2011,^[33] with Kay appearing as his alter ego Geraldine McQueen. Their version reached number 11 on the UK Singles Chart. A parody music video was also created.

"I Know Him So Well" was also covered by English recording artist Melanie C. The song was the first single^[34] to be taken from Melanie C's musical theatre-inspired and sixth studio album Stages. The song is a duet with Melanie C's fellow Spice Girls group member Emma Bunton. The song was released as a single on 11 November 2012.^[34] The b-side of the single is a cover of "You'll Never Walk Alone" from the musical Carousel.^[35] The music video for the song premiered on YouTube on 12 November 2012.

The song has been covered in non-charting versions by several notable performers.

• Originally recorded in 1985 during sessions for The Broadway Album, Barbra Streisand released her version featuring Mr. Mister's Richard Page on her 1991 box set Just for the Record...
• Pianist Richard Clayderman included an instrumental version on his 1987 album, Songs of Love.^[38]
• Singers John Barrowman and Daniel Boys covered the song on Barrowman's 2008 album Music Music Music.^[39]
• In 2014, Charlotte Jaconelli and Kerry Ellis performed the song on Jaconelli's solo debut album, Solitaire.^[40]
• Sheridan Smith sang the song with Amanda Holden on Holden's 2020 debut album Songs from My Heart.^[41]
• In 2022, YouTuber singer–songwriter AJ Rafael released a single version along with filmmaker and singer Chance Calloway to raise funds for The Trevor Project.^[42]