A choir () (also known as a quire, chorale or chorus) is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform. Choirs may perform music from the classical music repertoire, which spans from the Medieval era to the present, or popular music repertoire. Most choirs are led by a conductor, who leads the performances with arm and face gestures.

Composers of the late 18th century became fascinated with the new possibilities of the symphony and other instrumental music, and generally neglected choral music. Mozart's choral works, though not as numerous as his works for other media, stand out as some of his greatest (such as the "Great" Mass in C minor and Requiem in D minor, the latter of which is often considered the greatest Requiem Mass of all time). Haydn became more interested in choral music near the end of his life following his visits to England in the 1790s, when he heard various Handel oratorios performed by large forces; he wrote a series of masses beginning in 1797 and his two great oratorios The Creation and The Seasons. Beethoven wrote only two masses, both intended for liturgical use, although his Missa solemnis is probably suitable only for the grandest ceremonies due to its length, difficulty and large-scale scoring. He also pioneered the use of chorus as part of symphonic texture with his Ninth Symphony and Choral Fantasia.  In the 19th century, sacred music escaped from the church and leaped onto the concert stage, with large sacred works unsuitable for church use, such as Berlioz's Te Deum and Requiem, and Brahms's Ein deutsches Requiem. Rossini's Stabat mater, Schubert's masses, and Verdi's Requiem also exploited the grandeur offered by instrumental accompaniment. Oratorios also continued to be written, clearly influenced by Handel's models. Berlioz's L'enfance du Christ and Mendelssohn's Elijah and St Paul are in the category. Schubert, Mendelssohn, and Brahms also wrote secular cantatas, the best known of which are Brahms's Schicksalslied and Nanie.  A few composers developed a cappella music, especially Bruckner, whose masses and motets startlingly juxtapose Renaissance counterpoint with chromatic harmony. Mendelssohn and Brahms also wrote significant a cappella motets. The amateur chorus (beginning chiefly as a social outlet) began to receive serious consideration as a compositional venue for the part-songs of Schubert, Schumann, Mendelssohn, Brahms, and others. These 'singing clubs' were often for women or men separately, and the music was typically in four-part (hence the name "part-song") and either a cappella or with simple instrumentation. At the same time, the Cecilian movement attempted a restoration of the pure Renaissance style in Catholic churches.

What other works are notable?
such as the "Great" Mass in C minor and Requiem in D minor, the latter of which is often considered the greatest Requiem Mass of all time