Agnus Dei was originally written as the Adagio for Strings. It was later written to the text of Agnus Dei for choirs. The piece uses an arch form, employing and then inverting, expanding, and varying a stepwise ascending melody.

In 2004, Barber's masterpiece was voted the "saddest classical" work ever by listeners of the BBC's Today programme, ahead of "Dido's Lament" from Dido and Aeneas by Henry Purcell, and the "Adagietto" from Gustav Mahler's 5th symphony.

PC Gamers may recognize this piece from the opening sequence of Homeworld.