In organology, the study of musical instruments, many methods of classifying instruments exist. Most methods are specific to a particular cultural group and were developed to serve the musical needs of that culture. Culture-based classification methods sometimes break down when applied outside that culture. For example, a classification based on instrument use may fail when applied to another culture that uses the same instrument differently.

In the study of Western music, the most common classification method divides instruments into the following groups:

The criteria for classifying musical instruments vary according to point of view, time, and place. The many different approaches consider aspects such as the physical characteristics of the instrument (shape, construction, material composition, physical condition, etc.), the manner in which the instrument is played (plucked, bowed, etc.), the means by which the instrument produces sound, the quality or timbre of the sound produced by the instrument, the tonal and dynamic range of the instrument, the musical function of the instrument (rhythmic, melodic, etc.), and the place of the instrument in an orchestra or other ensemble.

2nd-century Greek grammarian, sophist, and rhetorician Julius Pollux, in the chapter called De Musica of his ten-volume Onomastikon, presented the two-class system, percussion (including strings) and winds, which persisted in medieval and postmedieval Europe. It was used by St. Augustine (4th and 5th centuries), in his De Ordine, applying the terms rhythmic (percussion and strings), organic (winds), and adding harmonic (the human voice); Isidore of Seville (6th to 7th centuries); Hugh of Saint Victor (12th century), also adding the voice; Magister Lambertus (13th century), adding the human voice as well; and Michael Praetorius (17th century).

The modern system divides instruments into wind, strings and percussion. It is of Greek origin (in the Hellenistic period, prominent proponents being Nicomachus and Porphyry). The scheme was later expanded by Martin Agricola, who distinguished plucked string instruments, such as guitars, from bowed string instruments, such as violins. Classical musicians today do not always maintain this division (although plucked strings are grouped separately from bowed strings in sheet music), but distinguish between wind instruments with a reed (woodwinds) and those where the air is set in motion directly by the lips (brass instruments).

Many instruments do not fit very neatly into this scheme. The serpent, for example, ought to be classified as a brass instrument, as a column of air is set in motion by the lips. However, it looks more like a woodwind instrument, and is closer to one in many ways, having finger-holes to control pitch, rather than valves.

Keyboard instruments do not fit easily into this scheme. For example, the piano has strings, but they are struck by hammers, so it is not clear whether it should be classified as a string instrument or a percussion instrument. For this reason, keyboard instruments are often regarded as inhabiting a category of their own, including all instruments played by a keyboard, whether they have struck strings (like the piano), plucked strings (like the harpsichord) or no strings at all (like the celesta).

It might be said that with these extra categories, the classical system of instrument classification focuses less on the fundamental way in which instruments produce sound, and more on the technique required to play them.