Viola is a string instrument, similar in construction to the violin, though somewhat longer. It forms the middle voice of the violin family, between the and the cello. Though extremely similar to the violin in pitch range and identical playing position, it is the rich timbre of the viola which sets it apart. It does not enjoy the popularity of the violin because its mellow voice is frequently used for playing inner harmonies.

In terms of material and construction the viola is quite similar to the violin, but it is larger in size and more variable in its proportions. The body of a full-size viola may be anywhere between 15 and 18 inches, with an average length of about 16 inches. It makes a less spectacular sound than the violin, being an instrument in the middle register, as a result of which it is less conspicuous. The highest notes do not make much of an impact. The bow of the viola is made of a wider band of horsehair than the violin and is therefore heavier. The profile of the outside corner of a viola bow frog generally is rounded, compared to the rectangular corner usually seen on violin bows.

The technique of playing the viola, though similar to the violin, has quite a few differences. Due to the difference in size, the notes are spread much farther along the fingerboard, as a result of which it requires different fingerings. The difference is also due to the strings being less responsive and the heavier bow. It is common for some players to use a wider and more intense vibrato in the left hand facilitated by employing the fleshier pad of the finger rather than the tip, and to hold the bow and right arm farther away from the player`s body. The player must also bring the left elbow farther forward or around, so as to reach the lowest string. This allows the fingers to be firm and create a clearer tone. Since the viola has thicker strings, it produces sound more slowly than its soprano cousin, the violin. Also more weight has to be applied with the bow in order to make noise. A person who plays the viola is called a violist.

The viola is extensively used in the world of Western Classical music. Jazz music has also seen its share of violists, from those used in string sections in the early 1900s to a handful of quartets and soloists emerging from the 1960s onward. Though not as common as the violin, the viola is nevertheless used by many folk musicians across the world.