Music term for a piece of equipment installed between the instrument (usually a guitar) and the amp to enhance or distort the sound. Examples of effect boxes are fuzzbox, distortion, overdrive, flanger, phaser, chorus, reverb, compressor, and many others.

With modern DSP units, multi-effect boxes with two or more effects in one box have become more common.

The term "effect box" was originally used for the small, foot-operated units used by guitar and bass players, but now the term also applies to rack-mounted equipment used for any kind of instrument.