Any one of a family of bowed, fretted stringed musical instruments developed in the 1400s and used primarily in the Renaissance and Baroque periods.

The family is related to and descends primarily from the Spanish vihuela (a guitar-like plucked string instrument). Some degree of developmental influence, if only in playing posture, is credited to the Moorish rebab as well.

Due to their comparatively large sizes, this new instrument was usually held upright, either resting on the lap or held between the legs, similar to the playing posture of a cello. This gave rise to its Italian name viola da gamba, meaning 'viol of the leg', which also helped differentiate it from the visually similar but only distantly related early violin family which the Italians called viola da braccio (lit. 'viol of the arm').

Viols most commonly had 6 strings, although many 16th century instruments had 5 or even 4 strings. Viols were (and are) strung with (low tension) gut strings, unlike the steel strings used by members of the modern violin family. Gut strings produce a sonority far different from steel, which is generally described as softer and sweeter. Around 1660, gut or silk core strings overspun with copper wire first became available. These were then used for the lowest pitched bass strings on viols (and on many other string instruments as well).

Viols are fretted in a manner similar to early guitars or lutes, by means of movable, wrapped-around and tied-on gut frets. Unlike members of the violin family, which are tuned in fifths, viols are usually tuned in fourths with a major third in the middle, mirroring the tuning employed on the vihuela de mano and lute during the 16th century, and similar to that of the modern 6-string guitar.