In music, a quartet (French: quatuor, German: Quartett, Italian: quartetto, Spanish: cuarteto, Polish: kwartet) is an ensemble of four singers or instrumental performers.

In classical music, one of the most common combinations of four instruments in chamber music is the string quartet. String quartets most often consist of two violins, a viola, and a cello. The particular choice and number of instruments derives from the registers of the human voice: soprano, alto, tenor and bass (SATB). In the string quartet, two violins play the soprano and alto vocal registers, the viola plays the tenor register and the cello plays the bass register.

Composers of notable string quartets include Joseph Haydn (68 compositions), Wolfgang Amadeus Mozart (23), Ludwig van Beethoven (16), Franz Schubert (15), Felix Mendelssohn (6), Johannes Brahms (3), Antonín Dvořák (14), Alexander Borodin (2), Béla Bartók (6), Elizabeth Maconchy (13), Darius Milhaud (18), Heitor Villa-Lobos (17), and Dmitri Shostakovich (15). The Italian composer Luigi Boccherini (1743–1805), wrote 91 string quartets.

Less often, string quartets are written for other combinations of the standard string ensemble. These include quartets for one violin, two violas, and one cello, notably by Carl Stamitz (6 compositions) and others; and for one violin, one viola, and two cellos, by Johann Georg Albrechtsberger and others.

Another common standard classical quartet is the piano quartet, usually consisting of violin, viola, cello, and piano. Romantic composers Beethoven, Brahms, and Mendelssohn each wrote three important compositions in this form, and Mozart, Dvořák, and Gabriel Fauré each wrote two. Olivier Messiaen's Quartet for the End of Time substitutes clarinet for viola.

Wind quartets are scored either the same as a string quartet with the wind instrument replacing the first violin (i.e. scored for wind, violin, viola and cello) or are groups of four wind instruments. Among the latter, the SATB format woodwind quartet of flute, oboe, clarinet, and bassoon is relatively common.

An example of a wind quartet featuring four of the same types of wind instruments is the saxophone quartet, consisting of soprano saxophone, alto saxophone, tenor saxophone and baritone saxophone. Often a second alto may be substituted for the soprano part or a bass saxophone may be substituted for the baritone.^{[citation needed]}

Compositions for four singers have been written for quartets a cappella; accompanied by instruments, such as a piano; and accompanied by larger vocal forces, such as a choir. Brahms and Schubert wrote numerous pieces for four voices that were once popular in private salons, although they are seldom performed today. Vocal quartets also feature within larger classical compositions, such as opera, choral works, and symphonic compositions. The final movement of Beethoven's Ninth Symphony and Giuseppe Verdi's Messa da Requiem are two examples of renowned concert works that include vocal quartets.