Bass (pronounced like the word "base") describes musical instruments that produce tones in the low-pitched range. They belong to different families of instruments and can cover a wide range of musical roles. Since producing low pitches usually requires a long air column or string, the string and wind bass instruments are usually the largest instruments in their families or instrument classes.

As seen in the musical instrument classification article, categorizing instruments can be difficult. For example, some instruments fall into more than one category. The cello is considered a tenor instrument in some orchestral settings, but in a string quartet it is the bass instrument.