An effects unit (effect box, stomp box, stompbox, or pedal) is an electronic device that alters how a musical instrument or other audio source sounds. Some effects subtly "color" a sound, while others transform it dramatically. Musicians use effects units during live performances or in the studio, typically with electric guitar, keyboard, or bass. While most frequently used with electric or electronic instruments, effects can also be used with acoustic instruments, drums and vocals. Examples of common effects units include wah-wah pedals, fuzzboxes and reverb units.

Effects are housed in amplifiers, table top units, "stompboxes" and "rackmounts", or they are built into the instruments themselves. A stompbox (or "pedal"—particularly if it features an actual pedal, as in a wah-wah pedal) is a small metal or plastic box placed on the floor in front of the musician and connected to the instrument and the instrument amplifier with patch cords. Typically, one or more on-off foot switches control a device that provides only one or two effects. A rackmount device mounts on a standard 19-inch equipment rack and usually contains several types of effects.