O come, O come, Emmanuel is a translation of the Catholic Latin text ("Veni, veni, Emmanuel") by John Mason Neale in the mid-19th century. It is a metrical version of a collation of various Advent Antiphons (the acrostic O Antiphons), which now serves as a popular Advent and Christmas hymn. Its origins are unclear, it is thought that the antiphons are from at least the 8th Century, but "Veni, veni Emmanuel" may well be 12th Century in origin. The text is based on the biblical prophecy from Isaiah 7:14 that states that God will give Israel a sign that will be called Immanuel (Lit.: God with us). Matthew 1:23 states fulfillment of this prophecy in the birth of Jesus of Nazareth.
It is believed that the traditional music stems from a 15th Century French processional for Franciscan nuns, but it may also have 8th Century Gregorian origins. It is one of the most solemn Advent hymns.
One widespread practice in the Catholic Church has two subsequent verses sung each week of Advent, beginning with the First Sunday of Advent as verses 1 & 2. The Second Sunday of Advent, verses 3 & 4 are sung. On the Third Sunday of Advent, verses 5 & 6. On the Fourth Sunday of Advent however, verses 1 & 7 are then sung.
Performance variations exist today over the rhythm of the music. Many performances pause after "Emmanuel" in both the verse and the chorus, or extend the final syllable through a similar count. Often however, performances omit these pauses to emphasize the meaning of the chorus: "Rejoice, rejoice, Emmanuel shall come to thee O Israel". If a pause is included, the meaning may be confused, as an audible comma is perceived between "Emmanuel" and "shall come to thee...", changing the grammatical subject of the sentence from Israel to Emmanuel. Rushing the first and final lines to omit the pause produces a greater sense of movement, which may or may not be desirable in performance as it contrasts with the unhurried pace of the remainder of the song.
Ottorino Respighi quotes the melody in "The Gift of the Magi" in his Trittico Botticelliano.
The composer James MacMillan wrote a percussion concerto based on this carol in 1991, and it was later premiered during the 1992 BBC Proms.
The composer Arvo Part wrote a symphony, his 1977 Symphony no. 3, that utilizes the melody and expresses the millenarian (or even apocalyptic) theme of the text. He also wrote seven Magnificat Antiphons, which were essentially the German texts of the hymn set to a variety of arrangements.
The popular rock group U2 also pays homage to the first haunting phrases of the hymn in the song "White as Snow" from their album "No Line on the Horizon." 
Recorded versions (a selection)