The Te Deum (also known as Te Deum Laudamus, Ambrosian Hymn or A Song of the Church) is an early Christian hymn of praise. The title is taken its opening Latin words, rendered literally as "Thee, O God, we praise".
The hymn remains in regular use in the Catholic Church in the Office of Readings found in the Liturgy of the Hours, and in thanksgiving to God for a special blessing such as the election of a pope, the consecration of a bishop, the canonization of a saint, a religious profession, the publication of a treaty of peace, a royal coronation, etc.It is sung either after Mass or the Divine Office or as a separate religious ceremony. The hymn also remains in use in the Anglican Communion and some Lutheran Churches in similar settings.
In the traditional Office, the Te Deum is sung at the end of Matins on all days when the Gloria is said at Mass; those days are all Sundays outside Advent, Septuagesima, Lent, and Passiontide; on all feasts (except the Triduum) and on all ferias during Eastertide. Before the 1962 reforms, neither the Gloria nor the Te Deum were said on the feast of the Holy Innocents, unless it fell on Sunday, as they were martyred before the death of Christ and therefore could not immediately attain the beatific vision.
In the Liturgy of the Hours of Paul VI, the Te Deum is sung at the end of the Office of Readings on days when the Gloria is sung (Sundays outside Advent and Lent), and all solemnities, including the octaves of Easter and Christmas, and all feasts) and also on the Sundays of Advent. It is also used together with the standard canticles in Morning Prayer as prescribed in the Anglican Book of Common Prayer, in Matins for Lutherans, and is retained by many other churches of the Reformed tradition. It is also used by the Eastern Orthodox Churches in the Paraklesis (Moleben) of Thanksgiving.
Authorship is traditionally ascribed to Saints Ambrose and Augustine, on the occasion of the latter's baptism by the former in AD 387. It has also been ascribed to Saint Hilary, but The Catholic Forum  says "it is now accredited to Nicetas, bishop of Remesiana; (4th century)".
The petitions at the end of the hymn (beginning Salvum fac populum tuum) are a selection of verses from the book of Psalms, appended subsequently to the original hymn.
The hymn follows the outline of the Apostles' Creed, mixing a poetic vision of the heavenly liturgy with its declaration of faith. Calling on the name of God immediately, the hymn proceeds to name all those who praise and venerate God, from the hierarchy of heavenly creatures to those Christian faithful already in heaven to the Church spread throughout the world. The hymn then returns to its creedal formula, naming Christ and recalling his birth, suffering and death, his resurrection and glorification. At this point the hymn turns to the subjects declaiming the praise, both the universal Church
The text has been set to music by many composers, with settings by Haydn, Mozart, Berlioz, Verdi, Bruckner, Dvořák, Britten, Kodály, and Pärt among the better known. Antonio Vivaldi also wrote a setting of the Te Deum (RV 622), but this composition is now lost. The prelude to Charpentier's setting (H.146 in Hugh Wiley Hitchcock's catalogue) is well known in Europe on account of its being used as the theme music for some broadcasts of the European Broadcasting Union, most notably the Eurovision Song Contest. Sir William Walton's Coronation Te Deum was written for the coronation of HM Queen Elizabeth II in 1952. Other English settings include those by William Byrd, Thomas Tallis, Henry Purcell, three versions by George Frideric Handel (Utrecht Te Deum and Jubilate, Dettingen Te Deum and Queen's Te Deum), and that of Edward Elgar, his Op. 34. Charles Villiers Stanford also wrote three settings: Te Deum in B flat, Op. 10; Te Deum in C, Op. 115; and Te Deum in A. Jean-Baptiste Lully wrote his own setting of Te Deum for the French Court, and eventually killed himself conducting it.
A version by Father Michael Keating is popular in some Charismatic circles. Mark Hayes wrote a setting of the text in 2005, with Latin phrases interpolated amid primarily English lyrics. British composer John Rutter has composed two settings of this hymn, one entitled Te Deum and the other Winchester Te Deum. Igor Stravinsky set the first 12 lines of the text as part of The Flood in 1962. Antony Pitts was commissioned by the London Festival of Contemporary Church Music to write a setting for the 2011 10th Anniversary Festival.