The harmonica, also known as a French harp or mouth organ, is a free-reed wind instrument used worldwide in many musical genres, notably in blues, American folk music, classical music, jazz, country, and rock. The many types of harmonica include diatonic, chromatic, tremolo, octave, orchestral, and bass versions. The harmonica is played by using the lips and tongue to direct air into or out of one (or more) holes along a mouthpiece (which covers one edge of the harmonica for most of its length). Behind each hole is a chamber containing at least one reed. The most common type of harmonica is a diatonic Richter-tuned instrument with ten air passages and twenty reeds, often called a blues harp. The harmonica reed is a flat, elongated spring typically made of brass, stainless steel, or bronze, which is secured at one end over a slot that serves as an airway. When the free end is made to vibrate by the player's air, the reed alternately blocks and unblocks the airway to produce sound.

Reeds are tuned to individual pitches. Tuning may involve changing a reed's length, how heavy it is near its free end, or how stiff it is near its fixed end. Reeds that are longer, heavier, or more flexible produce lower pitches; shorter, lighter, or stiffer reeds produce higher pitches. If, as on most modern harmonicas, a reed is affixed above or below its slot rather than in the plane of the slot, it responds more easily to air flowing in the direction that initially would push it into the slot, i.e., as a closing reed. This difference in response to air direction makes it possible to include both a blow reed and a draw reed in the same air chamber, and to play them separately without relying on flaps of plastic or leather (valves, wind-savers) to block the nonplaying reed.

An important technique in performance is bending, causing a drop in pitch by making embouchure adjustments. Bending the pitch of an isolated reed is possible on chromatic models (or any others that have wind-savers), but it is also possible to lower or raise (overbend, overblow, overdraw) the pitch produced by pairs of reeds in the same chamber, as on a diatonic or other unvalved harmonica. Such two-reed pitch changes actually involve sound production by the normally silent reed, the opening reed (for instance, the blow reed while the player is drawing).

The basic parts of the harmonica are the comb, reed plates, and cover plates.

The comb is the main body of the instrument, which, when assembled with the reed plates, forms air chambers for the reeds. The term "comb" may originate from the similarity between this part of a harmonica and a hair comb. Harmonica combs were traditionally made from wood, but now are also made from plastic (ABS) or metal (including titanium for high-end instruments). Some modern and experimental comb designs are complex in the way that they direct the air.

Dispute exists among players about whether comb material affects the tone of a harmonica. Those saying no argue that unlike the soundboard of a piano or the top piece of a violin or guitar, a harmonica's comb is neither large enough nor able to vibrate freely enough to substantially augment or change the sound. Among those saying yes are those who are convinced by their ears. Few dispute that comb surface smoothness and air tightness when mated with the reed plates can greatly affect tone and playability. The main advantage of a particular comb material over another one is its durability. In particular, a wooden comb can absorb moisture from the player's breath and contact with the tongue. This can cause the comb to expand slightly, making the instrument uncomfortable to play, and to then contract, potentially compromising air tightness. Various types of wood and treatments have been devised to reduce the degree of this problem.

An even more serious problem with wooden combs, especially in chromatic harmonicas (with their thin dividers between chambers), is that, as the combs expand and shrink over time, cracks can form in the combs, because the comb is held immobile by nails, resulting in disabling leakage. Serious players devote significant effort to restoring wood combs and sealing leaks. Some players used to soak wooden-combed harmonicas (diatonics, without wind-savers) in water to cause a slight expansion, which they intended to make the seal between the comb, reed plates, and covers more airtight. Modern wooden-combed harmonicas are less prone to swelling and contracting, but modern players still dip their harmonicas in water for the way it affects tone and ease of bending notes.

The reed plate is a grouping of several reeds in a single housing. The reeds are usually made of brass, but steel, aluminium, and plastic are occasionally used. Individual reeds are usually riveted to the reed plate, but they may also be welded or screwed in place. Reeds fixed on the inner side of the reed plate (within the comb's air chamber) respond to blowing, while those fixed on the outer side respond to suction.