A string quintet is a musical composition for five string players. As an extension to the string quartet (two violins, a viola, and a cello), a string quintet includes a fifth string instrument, usually a second viola (a so-called "viola quintet") or a second cello (a "cello quintet"), or occasionally a double bass.
Notable examples of classic "viola quintets", in four movement form include those of Wolfgang Amadeus Mozart. Other examples were written by composers including Johannes Brahms and Felix Mendelssohn.