The War Requiem, Op. 66, is a choral and orchestral composition by Benjamin Britten, composed mostly in 1961 and completed in January 1962. The War Requiem was performed for the consecration of the new Coventry Cathedral, in the English county of Warwickshire, which was built after the original fourteenth-century structure was destroyed in a World War II bombing raid. The traditional Latin texts are interspersed, in telling juxtaposition, with extra-liturgical poems by Wilfred Owen, written during World War I.

Britten scored the work for soprano, tenor and baritone soloists, chorus, boys' choir, organ, and two orchestras (a full orchestra and a chamber orchestra). The chamber orchestra accompanies the intimate settings of the English poetry, while soprano, choirs and orchestra are used for the Latin sections; all forces are combined in the conclusion. The Requiem has a duration of approximately 80–85 minutes. In 2019, Britten's 1963 recording of the War Requiem was selected by the Library of Congress for preservation in the National Recording Registry for being "culturally, historically, or aesthetically significant".

The War Requiem, first performed on 30 May 1962, was commissioned to mark the consecration of the new Coventry Cathedral, which was built after the original 14th-century structure was destroyed in a World War II bombing raid. The reconsecration was an occasion for an arts festival, for which Michael Tippett also wrote his opera King Priam.

Britten, a pacifist and conscientious objector, was inspired by the commission, which gave him complete freedom in deciding what to compose. He chose to set the traditional Latin Mass for the Dead interwoven with nine poems about war by the English poet Wilfred Owen. Owen, who was born in 1893, was serving as the commander of a rifle company when he was killed in action on 4 November 1918 during the crossing of the Sambre–Oise Canal in France, just one week before the Armistice. Although he was virtually unknown at the time of his death, he has subsequently come to be revered as one of the great war poets.

Philip Reed has discussed the progression of Britten's composition of the War Requiem in the Cambridge Music Handbook publication on the work. Britten himself acknowledged the stylistic influence of Requiems by other composers, such as Giuseppe Verdi's, on his own composition. David B. Greene has discussed Britten's 'indictment' of religious music in this work.

Britten dedicated the work to Roger Burney, Piers Dunkerley, David Gill, and Michael Halliday. Burney and Halliday, who died in the war, were friends of Peter Pears and Britten, respectively. According to the Britten-Pears Foundation's War Requiem website, Dunkerley, one of Britten's closest friends, took part in the 1944 Normandy landings. Unlike the other dedicatees, he survived the war but took his own life in June 1959, two months before his wedding. None of the other dedicatees have known graves, but are commemorated on memorials to the missing.

The musical forces are divided into three groups that alternate and interact with each other throughout the piece, finally fully combining at the end of the last movement. The soprano soloist and choir are accompanied by the full orchestra, the baritone and tenor soloists are accompanied by the chamber orchestra, and the boys' choir is accompanied by a small portative organ (this last group ideally being situated at some distance from the full orchestra). This group produces a very strange, distant sound. The soprano and choir and the boys' choir sing the traditional Latin Requiem text, while the tenor and baritone sing poems by Wilfred Owen, interspersed throughout.

The full orchestra consists of the following instrumentation.