A brass instrument is a musical instrument that produces sound by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips. The term labrosone, from Latin elements meaning "lip" and "sound", is also used for the group, since instruments employing this "lip reed" method of sound production can be made from other materials like wood or animal horn, particularly early or traditional instruments such as the cornett, alphorn or shofar.

There are several factors involved in producing different pitches on a brass instrument. Slides, valves, crooks (though they are rarely used today), or keys are used to change vibratory length of tubing, thus changing the available harmonic series, while the player's embouchure, lip tension and air flow serve to select the specific harmonic produced from the available series.

The view of most scholars (see organology) is that the term "brass instrument" should be defined by the way the sound is made, as above, and not by whether the instrument is actually made of brass. Thus one finds brass instruments made of wood, like the alphorn, the cornett, the serpent and the didgeridoo, while some woodwind instruments are made of brass, like the saxophone.

Modern brass instruments generally come in one of two families:

There are two other families that have, in general, become functionally obsolete for practical purposes. Instruments of both types, however, are sometimes used for period-instrument performances of Baroque or Classical pieces. In more modern compositions, they are occasionally used for their intonation or tone color.

Brass instruments may also be characterised by two generalizations about geometry of the bore, that is, the tubing between the mouthpiece and the flaring of the tubing into the bell. Those two generalizations are with regard to

While all modern valved and slide brass instruments consist in part of conical and in part of cylindrical tubing, they are divided as follows:

The resonances of a brass instrument resemble a harmonic series, with the exception of the lowest resonance, which is significantly lower than the fundamental frequency of the series that the other resonances are overtones of. Depending on the instrument and the skill of the player, the missing fundamental of the series can still be played as a pedal tone, which relies mainly on vibration at the overtone frequencies to produce the fundamental pitch. The bore diameter in relation to length determines whether the fundamental tone or the first overtone is the lowest partial practically available to the player in terms of playability and musicality, dividing brass instruments into whole-tube and half-tube instruments. These terms stem from a comparison to organ pipes, which produce the same pitch as the fundamental pedal tone of a brass instrument of equal length.