Bass (pronounced like the word "base") is a family of 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.

Examples grouped by general form and playing technique include:

• Bass guitar and acoustic bass guitar, instruments shaped, constructed and held (or worn) like guitars, that play in the bass range
• Double bass from the viol or violin family (usually the instrument referred to as a "bass" in European classical music and jazz. Sometimes called a "string bass" to differentiate it from a "bass horn")
• A bass horn, such as a tuba, serpent, and sousaphone from the wind family and low-tuned versions of specific types of brass and woodwind instruments, such as bassoon, bass clarinet, bass trombone and bass saxophone, etc. (less common usage)
• Keyboard bass, a keyboard alternative to the bass guitar or double bass (e.g. the Fender Rhodes piano bass in the 1960s or 13-note MIDI keyboard controllers in the 2000s)
• Washtub bass, a simple folk instrument
• Bass drum

A musician playing one of these instruments is often known as a bassist. Other more specific terms such as 'bass guitarist', 'double bassist', 'bass player', etc. may also be used.