Distortion pedals are a type of effects unit designed to add distortion to an audio signal to create a warm, gritty, or fuzzy character. Depending on the style of signal clipping, the broader category of distortion pedals is typically divided into fuzz pedals, distortion pedals, and overdrive pedals. Designed for electric guitar and bass and operated by the player's foot, distortion pedals are most frequently placed in the signal chain between the guitar and amplifier.

The use of distortion pedals was popularized by Keith Richards' use of a Maestro FZ-1 Fuzz-Tone pedal on the 1965 Rolling Stones song "(I Can't Get No) Satisfaction". Later pedals like the Pro Co Rat and Ibanez Tube Screamer have achieved iconic status among guitarists and are a key element in many players' tones. Industry publications often publish lists of influential and popular models.

While "distortion" is the technical term for the different sounds caused by signal clipping, distortion effects pedals—which act as small amplifier circuits—are typically further classified into fuzz, distortion, and overdrive, depending on the circuit design and how the waveform is affected. Fuzz pedals came to the market first, with these pedals seeking to replicate the harmonic content generated by an amplifier being faulty or damaged. This sound is often characterized as harsh, with the waveform the most heavily affected of the three types, reaching nearly a square form. To create their effect, fuzz pedals use lo-fi transistors.

Conventional overdrives, like the Ibanez Tube Screamer, use diodes within the feedback loop of an op amp, so the signal does not fully distort or clip and results in a milder effect. This is known as "soft-clipping," or overdrive. The affected waveform maintains rounded peaks with the intent to emulate and/or complement a traditional vacuum tube-based amplifier. Some overdrives are meant to "color" the affected tone, while others are meant to be relatively transparent, adding drive but otherwise mostly preserving the guitar and amp's basic character. When the diodes are placed at the end of the circuit, however, the entirety of the signal is clipped. This is known as "hard-clipping," or distortion, which has a more aggressive sound and dramatic effect on the underlying tone, with angular peaks in the affected waveform. Because overdrive and distortion pedals both typically use op amps and diodes, the line between the two types can blur. The Klon Centaur, for example, is hard-clipping but considered a classic overdrive, while the MXR Distortion + and DOD Overdrive 250 Preamp have nearly identical (hard-clipping) circuits despite the opposing use of "Distortion" and "Overdrive" in their names.

Some overdrive and distortion pedals, like the early fuzzes, use transistors and therefore do not have soft- or hard-clipping circuitry. These transistors are arranged in stages, cascading into one another for a more saturated sound, in a similar fashion to overdriving multiple vacuum tubes in an amp. Examples include the Boss Blues Driver and "amp in a box" overdrives like the Marshall JTM45-emulating Z.Vex Box of Rock.

Arbiter Electronics released the first Fuzz Face in 1966, featuring a unique round metal housing inspired by a microphone stand and with the arrangement of volume knob, distortion knob, and logo intentionally resembling a face. The Fuzz Face's first production run lasted until 1976/77, then was reissued from 1986 until 1990. In 1993, Dunlop Manufacturing took over production and has continued to issue several varieties of the pedal. Jimi Hendrix popularized the Fuzz Face and was known to buy multiple copies at a time to identify the best ones; his pedal chain often consisted of a wah-wah pedal into a Fuzz Face, then into a Uni-Vibe. Other notable users include Duane Allman, Stevie Ray Vaughan Pete Townshend, Eric Johnson, and George Harrison.

The Big Muff Pi (π), often known simply as the Big Muff, is a fuzz pedal produced in New York City by the Electro-Harmonix company, along with their Russian sister company Sovtek, primarily for use with the electric guitar. Released in 1969 and designed with an emphasis on long sustain compared to existing fuzzes, the Big Muff Pi was the first overwhelming success for Electro-Harmonix's line of pedals. Due to its reliability, its low price, and its distinctive sound, the Big Muff sold consistently through the 1970s and was found in numerous guitarists' pedal collections, including David Gilmour and Carlos Santana. The "sustained grind" of the Big Muff was later an integral part of the sound of many alternative rock bands through the 1980s and 1990s, being used extensively by the Smashing Pumpkins, Dinosaur Jr., NOFX, Bush and Mudhoney.

The Maestro FZ-1 Fuzz-Tone was the first widely marketed fuzz distortion guitar and bass effect. Introduced in 1962, it achieved widespread popularity in 1965 after the Rolling Stones guitarist Keith Richards prominent use of the FZ-1 on the group's hit, "(I Can't Get No) Satisfaction". Later in 1965, the design was slightly modified and designated as the model FZ1-1a. In the wake of the success of "Satisfaction," numerous recordings, mostly from 1960s garage rock and psychedelic acts, featured Maestro Fuzz-Tones on guitar. More significant design changes made after 1967 resulted in different renditions of the Fuzz-Tone released over the years. In the 1990s, Gibson re-issued the FZ-1a, but it was later discontinued.