Session musicians are musicians available for hire, as opposed to musicians who are either permanent members of a musical outfit or who have acquired fame in their own right. Although the term generally refers to musicians skilled in contemporary musical styles such as rock, jazz, country, pop, etc., it can also be used to describe musicians from classical disciplines. "Studio musicians" are session musicians who do not work outside of the recording studio environment.

Session musicians are used in any situation where musical skills are needed on a short-term basis, ranging from a few hours to many months. Typical uses of session musicians are:

• in a recording studio to provide backing for advertising, film and television music
• to provide musical or vocal backing for an established solo artist, both in the studio and in concerts and appearances
• to act as temporary replacements for permanent members of a band or ensemble
• to provide additional instruments or vocals when needed by a band or ensemble
• in ensembles for theatrical productions

Apart from being competent at their instruments, skilled session musicians are also expected to learn parts rapidly, and to play music by sight (reading directly from the musical score).

Many session musicians have long and successful careers. Although session musicians do not usually achieve popular celebrity (two notable exceptions include the members of the band Toto, formed by a group of regular studio musicians in Los Angeles, and Led Zeppelin, in which John Paul Jones and Jimmy Page were well known studio musicians prior to the band's formation), some achieve considerable fame within the musical industry. Examples include:

• (keyboardist) Greg Phillinganes who has worked with Michael Jackson, Aretha Franklin and Stevie Wonder,
• (drummer) Vinnie Colaiuta who has worked with Sting, Frank Zappa and Madonna
• (guitarist) the late Tommy Tedesco who was described by Guitar Player magazine as the most recorded guitarist in history, having played on thousands of recordings that include the musical themes for Bonanza and M*A*S*H*.