Agnus Dei
Agnus Dei is the Latin name under which the "Lamb of God" is honoured within Christian liturgies descending from the historic Latin liturgical tradition, including those of Roman Catholicism, Lutheranism and Anglicanism. It is the name given to a specific prayer that occurs in these liturgies, and is the name given to the music pieces that accompany the text of this prayer.
The use of the title "Lamb of God" in liturgy is based on John 1:29, in which St. John the Baptist, upon seeing Jesus, proclaims "Behold, the Lamb of God, who takes away the sin of the world!"

Liturgical usage
Latin Catholic
The Syrian custom of a chant addressed to the Lamb of God was introduced into the Roman Rite Mass by Pope Sergius I (687–701) in the context of his rejection of the Council of Trullo of 692 (which was well received in the Byzantine East), whose canons had forbidden the iconographic depiction of Christ as a lamb instead of a man.
The verse used in the first and second invocations may be repeated as many times as necessary whilst the celebrant prepares the host and wine for communion.In a Tridentine Requiem Mass, the words "miserere nobis" are replaced by "dona eis requiem" (grant them rest), while "dona nobis pacem" is replaced by "dona eis requiem sempiternam" (grant them eternal rest).
The priest uses the phrase "Lamb of God" again, later in the Mass. While displaying the Eucharistic species to the people before giving them Holy Communion, he says: "Ecce Agnus Dei, ecce qui tollit peccata mundi. Beati qui ad cenam Agni vocati sunt." ("Behold the Lamb of God, behold Him who takes away the sins of the world. Blessed are those called to the supper of the Lamb."): 132

Anglican
The following instances are found in the Church of England's Book of Common Prayer:

From "The Litany":

From "Holy Communion":

 

The following versions are found in Common Worship, the alternative Anglican liturgical resources, and also in the Episcopal Church's liturgical resources:

Lutheran
The version found in the Lutheran Service Book of the Lutheran Church–Missouri Synod is:

Evangelical Protestants
A popular worship song among Evangelical Protestants tradition is a rendition of the traditional Angus Dei by Michael W. Smith.  Based on a paraphrase of Revelation 5, the words are 

(Last stanza repeats 3 times)
A Ukrainian translation was sung in 2015 in Lviv, Ukraine, during the War in the Donbass at an event organized by Franklin Graham.  The song was notably sung again on Easter 2022 in Lviv by a choir of Ukrainian refugees fleeing the Russo-Ukrainian War.

Musical settings
Religious music
Virtually every Mass setting (of which there are thousands) includes an Agnus Dei. Here are some examples:

Machaut's Messe de Nostre Dame
Bach's Mass in B minor
Beethoven's Missa solemnis
Karl Jenkins' The Armed Man –  a mass for peace
Schubert's Mass No. 2
Bob Chilcott's Little Jazz Mass
Ralph Vaughan Williams's Mass in G minorWith a slightly changed text, the Agnus Dei is also part of musical settings composed for the Requiem Mass for the Dead. Such settings include:

Mozart's Requiem
Verdi's Requiem
Fauré's Requiem
Rutter's Requiem
Penderecki's Polish Requiem
Britten's War Requiem, in which the text is interleaved with Wilfred Owen's poem "At a Calvary near the Ancre"Some composers set the text as an independent movement, such as Samuel Barber, who wrote a version combining the text with the music of his Adagio for Strings, sung a cappella.

In popular culture
Outside of religious use, the text has been used by composers such as:

Elliot Goldenthal for Alien 3
Keiki Kobayashi for Ace Combat 04: Shattered Skies
Elitsa Alexandrova for Assassin's Creed Rogue
Enya for the song "Trains and Winter Rains"
Halsey for the song Castle
Kuroshitsuji (Black Butler) soundtrack for episodes 17 and 18
Jon Bellion for Ooh
New Age Worldbeat group Enigma (German band) for the song Agnus Dei
Rufus Wainright for the song Agnus Dei from the album Want Two
Mylène Farmer "Agnus Dei" (1991)
Dream Theater for the song Bridges in the sky from the album a dramatic turn of events
Paul Ruskay for Homeworld
Michael W. Smith "Agnus Dei" (1990)


== References ==