The Grande Messe des morts, Op. 5 (or Requiem) by Hector Berlioz was composed in 1837. The Grande Messe des Morts is one of Berlioz's best-known works, with a tremendous orchestration of woodwind and brass instruments, including four antiphonal offstage brass ensembles placed at the corners of the concert stage. The work derives its text from the traditional Latin Requiem Mass. It has a duration of approximately ninety minutes, although there are faster recordings of under seventy-five minutes.
In 1837, Adrien de Gasparin, the Minister of the Interior of France, asked Berlioz to compose a Requiem Mass to remember soldiers who died in the Revolution of July 1830. Berlioz accepted the request, having already wanted to compose a large orchestral work. Meanwhile, the orchestra was growing in size and quality, and the use of woodwinds and brass was expanding due to the increasing ease of intonation afforded by modern instruments. Berlioz later wrote, "if I were threatened with the destruction of the whole of my works save one, I should crave mercy for the Messe des morts." 
The premiere was conducted by François Antoine Habeneck on 5 December 1837 in commemoration of General Damrémont and the soldiers killed in the Siege of Constantine. In his autobiographical Mémoires, Berlioz claimed that Habeneck put down his baton during the dramatic Tuba mirum (part of the Dies irae movement) while he took a pinch of snuff,  prompting the composer to rush to the podium to conduct the rest of the work himself, thereby saving the performance from disaster. The premiere was a complete success. 
Berlioz revised the work twice in his life, first in 1852, making the final revisions in 1867, only two years before his death.
Berlioz's Requiem has ten movements, and the structure is as follows:
- 1. Requiem aeternam & Kyrie: Introitus
- 2. Dies irae: Prosa, Tuba mirum
- 3. Quid sum miser
- 4. Rex tremendae
- 5. Quaerens me
- 6. Lacrymosa
- 7. Domine Jesu Christe
- 8. Hostias
- 9. Sanctus
- 10. Agnus Dei
The Requiem is scored for a very large orchestra, four offstage brass bands, and chorus placed throughout the hall:
In relation to the number of singers and strings, Berlioz indicates in the score that, "The number [of performers] indicated is only relative. If space permits, the chorus may be doubled or tripled, and the orchestra be proportionally increased. But in the event of an exceptionally large chorus, say 700 to 800 voices, the entire chorus should only be used for the Dies Irae, the Tuba Mirum, and the Lacrymosa, the rest of the movements being restricted to 400 voices."
The work premiered with over four hundred performers.
The Requiem opens gravely with rising scales in the strings, horns, oboes, and cor anglais preceding the choral entry. Later, the music becomes extremely agitated with despair. The first movement contains the first two sections of the music for the Mass (the Introit and the Kyrie).
The Sequence commences in the second movement, with the Dies irae portraying Judgement Day. The four brass ensembles at the corners of the stage first appear in this movement, one by one; they are joined by sixteen timpani, two bass drums, and four tam-tams. The loud flourish is followed by the choral entry. There is a powerful unison statement by the basses, followed by the choir. Woodwinds and strings end the movement.
The third movement, Quid sum miser, is short, depicting after Judgement Day. It features an interesting orchestration of TTB chorus, two cors anglais, eight bassoons, cellos, and double basses. The Rex tremendae contains contrasting opposites. The choir sings both beseechingly, as if for help, and majestically. Quaerens me is a quiet, soft movement which is completely a cappella.
The sixth movement, Lacrimosa, is in 9/8 time signature, and is considered the center of the entire Requiem. It is the only movement written in recognizable sonata form and the last movement depicting pain. The dramatic effect of this movement is heightened by the addition of the massed brass and percussion. This movement concludes the Sequence section of the Mass.
The seventh movement begins the Offertory. Domine Jesu Christe is based on a repeated three-note motif: A, B flat, and A. The choral statements of this motive interweave with the orchestral melodies. The "A, B flat, A" motif persists for about ten minutes almost to the end, which concludes peacefully. Robert Schumann was very interested in the innovativeness of this movement. The concluding part of the Offertory, the Hostias, is short and scored for the male voices, eight trombones, three flutes, and strings.
A solo male tenor voice is featured in the ninth movement, the Sanctus. There are long held notes played by the flute. Women's voices also sing, perhaps answering the tenor. Later, the low strings and cymbals join in. A full orchestral fugue ends the movement. In his original version, Berlioz requested ten tenors for the solo part. The final movement, containing the Agnus Dei and Communion sections of the Mass, features long held chords by the woodwinds and strings. The movement recapitulates melodies and effects from previous movements.
- Leonard Bernstein conducts the Orchestre National de France and the Choeurs de Radio France with Stuart Burrows, Tenor. Recorded at St Louis des Invalides, Paris, 1975.
- Sir Thomas Beecham conducting the Royal Philharmonic Orchestra, Royal Philharmonic Chorus and Richard Lewis (tenor).
- Sir Colin Davis conducting the London Symphony Orchestra, London Symphony Chorus and Ronald Dowd (tenor)
- James Levine conducting the Berliner Philharmoniker, Ernest-Senff-Chor and Luciano Pavarotti.
- Charles Münch conducting the Boston Symphony Orchestra, New England Conservatory Chorus and Leopold Simoneau (tenor).
- Charles Münch conducting the Bavarian Radio Symphony Orchestra, Bavarian Radio Choir and Peter Schreier, Tenor. Recorded at Herkulessaal, Munich, July 1967.
- Seiji Ozawa conducting the Boston Symphony Orchestra, Tanglewood Festival Chorus and Vinson Cole.
- André Previn conducting the London Philharmonic Choir, London Philharmonic Orchestra and Robert Tear.
- Robert Shaw conducting the Atlanta Symphony Orchestra & Chorus and John Aler.
- Sir Colin Davis conducting the London Symphony Orchestra, London Symphony Chorus, London Philharmonic Choir and Barry Banks (tenor). St. Paul's Cathedral, June 2012.
- Steinberg, Michael. "Hector Berlioz: Requiem." Choral Masterworks: A Listener's Guide. Oxford: Oxford University Press, 2005, 61-67.
- Niecks, Frederick. "Berlioz's Messe des Morts and Its Performance in Glasgow". The Musical Times and Singing Class Circular 25, no. 493 (1 March 1884): 129–31.
- Overview of the Requiem including history, a description of the movements, and the complete text
- Requiem: Free scores at the International Music Score Library Project
- Free scores of this work in the Choral Public Domain Library (ChoralWiki)
- "The Berlioz Requiem - Pre-Concert Talk", lecture by David Cairns at Gresham College on 12 July 2007