A string trio is a group of three string instruments or a piece written for such a group. From at least the 19th century on, the term "string trio" with otherwise unspecified instrumentation normally refers to the combination violin, viola and cello. The classical string trio emerged during the mid-18th century and later expanded into four subgenres: the grand trio, the concertant trio, the brilliant trio, and the Hausmusik trio.

The earliest string trio, found during the mid 18th century, consisted of two violins and a cello, a grouping which had grown out of the Baroque trio sonata. Over the course of the late 18th century, the string trio scored for violin, viola, and cello came to be the predominant type.^[1] String trios scored for two violins and viola were also used, although much less frequently.^[2] The term "string trio" was not used until the late 19th century. The genre was previously referred to as divertimento a tre, sonate a tre, and terzetto.^[2] French composers sometimes used Trietti or Conversazioni a tré to name their trios.^[3] The early classical string trio generally consisted of three movements, although four, five and six movement trios were later written.^[1] Most movements were in binary form, although some of Joseph Haydn's trios were written as theme and variations. The violins shared the important thematic material, while the cello maintained an accompanimental role.^[3]

During the mid to late 18th century, a couple of developments occurred in the genre. The changes included the absence of a figured bass, the equal treatment of all voices (as opposed to the top voice dominating the musical texture), and the use of sonata form in the first movement.^[3] These early developments paved the way for the genre to further expand into four specific types: the grand trio, the concertant trio, the brilliant trio, and the Hausmusik trio.^[3]

• String quintet
• String sextet
• String octet