Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may be used with other instruments, such as electric bass, electric piano, synthesizer, and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. Other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, grunge and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

The effects alter the instrument sound by clipping the signal (pushing it past its maximum, which shears off the peaks and troughs of the signal waves), adding sustain and harmonic and inharmonic overtones and leading to a compressed sound that is often described as "warm" and "dirty", depending on the type and intensity of distortion used. The terms distortion and overdrive are often used interchangeably; where a distinction is made, distortion is a more extreme version of the effect than overdrive. Fuzz is a particular form of extreme distortion originally created by guitarists using faulty equipment (such as a misaligned valve (tube); see below), which has been emulated since the 1960s by a number of "fuzzbox" effects pedals.

Distortion, overdrive, and fuzz can be produced by effects pedals, rackmounts, pre-amplifiers, power amplifiers (a potentially speaker-blowing approach), speakers and (since the 2000s) by digital amplifier modeling devices and audio software. These effects are used with electric guitars, electric basses (fuzz bass), electronic keyboards, and more rarely as a special effect with vocals. While distortion is often created intentionally as a musical effect, musicians and sound engineers sometimes take steps to avoid distortion, particularly when using PA systems to amplify vocals or when playing back prerecorded music.

The first guitar amplifiers were relatively low-fidelity, and would often produce distortion when their volume (gain) was increased beyond their design limit or if they sustained minor damage. From 1935, Western swing guitarist Bob Dunn began experimenting with a distorted or "dirty" tone. Later, around 1945, Western swing guitarist and member of the Bob Wills band, Junior Barnard, began experimenting with a rudimentary humbucker pick-up and a small amplifier to obtain his signature "low-down and dirty" bluesy sound which allowed for more "fluid and funky" chords. Many electric blues guitarists, including Chicago bluesmen such as Elmore James and Buddy Guy, experimented to get a guitar sound that paralleled the rawness of blues singers such as Muddy Waters and Howlin' Wolf, replacing often their originals with the powerful Valco "Chicagoan" pick-ups, originally created for lap-steel, to obtain a louder and fatter tone. In early rock music, Goree Carter's "Rock Awhile" (1949) featured an over-driven electric guitar style similar to that of Chuck Berry several years later, as well as Joe Hill Louis' "Boogie in the Park" (1950).

In the early 1950s, guitar distortion sounds started to evolve based on sounds created earlier in the decade by accidental damage to amps, such as in the popular early recording of the 1951 Ike Turner and the Kings of Rhythm song "Rocket 88", where guitarist Willie Kizart used a vacuum tube amplifier that had a speaker cone slightly damaged in transport. Electric guitarists began "doctoring" amplifiers and speakers to emulate this form of distortion.

Electric blues guitarist Willie Johnson of Howlin' Wolf′s band began deliberately increasing gain beyond its intended levels to produce "warm" distorted sounds. Guitar Slim also experimented with distorted overtones, which can be heard in his hit electric blues song "The Things That I Used to Do" (1953). Chuck Berry's 1955 classic "Maybellene" features a guitar solo with warm overtones created by his small valve amplifier. Pat Hare produced heavily distorted power chords on his electric guitar for records such as James Cotton's "Cotton Crop Blues" (1954) as well as his "I'm Gonna Murder My Baby" (1954), creating "a grittier, nastier, more ferocious electric guitar sound," accomplished by turning the volume knob on his amplifier "all the way to the right until the speaker was screaming."

In 1956, guitarist Paul Burlison of the Johnny Burnette Trio deliberately dislodged a vacuum tube in his amplifier to record "The Train Kept A-Rollin" after a reviewer raved about the sound Burlison's damaged amplifier produced during a live performance. According to other sources Burlison's amp had a partially broken loudspeaker cone. Pop-oriented producers were horrified by that eerie "two-tone" sound, quite clean on trebles but strongly distorted on basses, but Burnette insisted on releasing the sessions, arguing that "that guitar sounds like a nice horn section".

In the late 1950s, guitarist Link Wray began manipulating his amplifiers' vacuum tubes to create a "noisy" and "dirty" sound for his solos after a similarly accidental discovery. Wray also poked holes in his speaker cones with pencils to further distort his tone, used electronic echo chambers (then usually employed by singers), the recent powerful and "fat" Gibson humbucker pickups, and controlled "feedback" (Larsen effect). The resultant sound can be heard on his highly influential 1958 instrumental, "Rumble" and Rawhide.