The Requiem (from Latin requiem, accusative case of requies, rest) or Requiem Mass (informally, a funeral Mass), also known formally (in Latin) as the Missa pro defunctis or Missa defunctorum, is a liturgical service of the Roman Catholic Church, Anglo-Catholic Anglicans, as well as certain Lutheran Churches in the United States. There is also a requiem, with a wholly different ritual form and texts, that is observed in the Eastern Orthodox and Eastern Catholic Churches. The common theme of requiems is prayer for the salvation of the soul(s) of the departed, and it is used both at services immediately preceding a burial, and on occasions of more general remembrance.

"Requiem" is also the title of various musical compositions used in such liturgical services or as concert pieces as settings of the portions of that Mass which have been traditionally sung in the Roman Catholic liturgy.

While the prayers in the regular Mass as the Introit and Gradual change according to the Calendar of Saints, the text for the requiem Mass is particularly fixed. Originally such funeral musical compositions were meant to be performed in liturgical service, with monophonic chant. Eventually the dramatic character began to appeal to composers to an extent that made the requiem a genre of its own.

The Requiem by Giuseppe Verdi is a musical setting of the Catholic funeral Mass (called the Requiem from the first word of the text, which begins Requiem aeternam dona eis, Domine, meaning, "Grant them eternal rest, O Lord"—see the entry at "Dies Irae"). It was completed to mark the first anniversary of the death of Alessandro Manzoni, an Italian poet and novelist much admired by Verdi. The piece is also sometimes referred to as the Manzoni Requiem.

The Requiem Mass in D minor (K. 626) by Wolfgang Amadeus Mozart was composed in 1791. The requiem was Mozart's last composition, and is one of his most powerful and recognized works , not only for its music, but also for the debate over how much of the music Mozart managed to complete before his death, and how much was later composed by his colleague Franz Xaver Süssmayr.

Despite debate about how much of the music was Mozart's, the Requiem has taken a prominent place as one of Mozart's most important works.

Gabriel Fauré composed his Requiem in D minor, Op. 48 between 1877 and 1890. This choral–orchestral setting of the Roman Catholic Mass for the Dead is the best known of his large works. The most famous movement is the soprano aria Pie Jesu. Camille Saint-Saëns said of it, "just as Mozart's is the only Ave verum Corpus, this is the only Pie Jesu."