A brass instrument is a musical instrument that uses a cupped mouthpiece shaped in a way that allows the player's lips to vibrate to generate the instrument's sound. Brass instruments are usually, but not invariably, made of brass. Similarly, in musical terms, not all instruments constructed from brass belong in the category of "brass instruments"; a notable example is the saxophone, which, although usually made of brass, is a woodwind instrument.

Because the player has direct control of the prime vibrator (the lips), brass instruments exploit the player's ability to select the harmonic at which the instrument's column of air will vibrate. By making the instrument about twice as long as the equivalent woodwind instrument and starting with the second harmonic, players can get a good range of notes simply by varying the tension of their lips (see embouchure).

Brass instruments generally come in one of three families:

• Natural brass instruments, where the player can only play notes in the instrument's harmonic series, for example the bugle.
• Valved brass instruments use a set of valves (3 or 4) operated by the player's fingers that introduce a additional lengths of tubing into the instrument, changing its overall length. This family includes the trumpet, cornet, flugelhorn, saxhorn, euphonium, tuba, Sousaphone, and French horn. The valves are usually piston valves, but can be rotary values. Rotary valves are the norm for the French horn.
• Slide brass instruments use a slide to change the length of tubing. The main instrument in this famility is the trombone and its ancestor the sackbut. Some modern trombones also have rotary valves in addition to the slide.

The trumpet, trombone, French horn, and tuba are the instruments most often found in a symphony orchestra.

For a comparative list of the pitch of various brass instruments see pitch of brass instruments

Some other brass instruments:

• alphorn (wood)
• shofar (horn)