The harmonica is a free reed wind instrument which is played by blowing air into it or drawing air out by placing lips over individual holes (reed chambers) or multiple holes.The pressure caused by blowing or drawing air into the reed chambers causes a reed or multiple reeds to vibrate up and down creating sound.Each chamber has multiple, variable-tuned brass or bronze reeds which are secured at one end and loose on the other end, with the loose end vibrating and creating sound.

Reeds are pre-tuned to individual tones, and each tone is determined according to the size of reed. Longer reeds make deep,low sounds and short reeds make higher-pitched sounds. On certain types of harmonica the pre-tuned reed can be changed (bending a note) to another note by redirecting air flow into the chamber.There are many types of harmonicas, including diatonic, chromatic, tremolo, orchestral, and bass versions.

The harmonica is used in blues and American folk music, jazz, classical music, country music, rock and roll, and pop music. The harmonica has other nicknames, especially in blues music, including: "harp", "blues harp", "french harp", and "mouth organ.