An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar, an electric bass, electric organ, electric piano, synthesizers and drum machine to convert the signal from the pickup (with guitars and other string instruments and some keyboards) or other sound source (e.g, a synthesizer's signal) into an electronic signal that has enough power, produced by a power amplifier, to drive one or more loudspeaker that can be heard by the performers and audience.

Combination (combo) amplifiers include a preamplifier, a power amplifier, tone controls, and one or more speakers in a cabinet, a housing or box usually made of wood. Instrument amplifiers for some instruments are also available without an internal speaker; these amplifiers, called heads, must plug into one or more separate speaker cabinets. Instrument amplifiers also have features that let the performer modify the signal's tone, such as changing the equalization (adjusting bass and treble tone) or adding electronic effects such as intentional distortion or overdrive, reverb or chorus effect.

Instrument amplifiers are available for specific instruments, including the electric guitar, electric bass, electric and electronic keyboards, and acoustic instruments such as the mandolin and banjo. Some amplifiers are designed for specific styles of music, such as the Fender tweed guitar amplifiers, such as the Fender Bassman used by blues and country music musicians, and the Marshall amplifiers used by hard rock and heavy metal bands.

Unlike home hi-fi amplifiers or public Address systems, which are designed to accurately reproduce the source sound signals with as little distortion as possible, instrument amplifiers are often designed to add additional tonal coloration to the original signal, and in many cases intentionally add some degree of distortion.

A guitar amplifier amplifies the electrical signal of an electric guitar so that it can drive a loudspeaker at sufficient volume for the performer and audience to hear. Most guitar amplifiers can also modify the instrument's sound with controls that emphasize or de-emphasize certain frequencies and add electronic effects. String vibrations are sensed by a pickup. For electric guitars, strings are made of metal, and the pickup works by electromagnetic induction.

Standard amplifiers, such as the Fender tweed-style amps (e.g., the Fender Bassman) are often used by traditional rock, blues, and country musicians who wish to create a vintage 1950s-style sound. They are used by electric guitarists, pedal steel guitar players, and blues harmonica players. These amps are designed to produce a variety of sounds ranging from a clean, warm sound to a growling, natural overdrive. These amplifiers usually have a sharp treble roll-off at 5 kHz to reduce the extreme high frequencies, and a bass roll-off at 60–100 Hz to reduce unwanted boominess. The nickname tweed refers to the lacquered beige light-brown fabric covering used on these amplifiers.

Combo amplifiers such as the Fender Super Reverb have powerful tube amplifiers, 10- or 12-inch speakers, and they often have built-in reverb and vibrato effects units. These larger combo amplifiers are used for club performances and larger venues. For large concert venues such as stadiums, performers may also use an amplifier head with several separate speaker cabinets. Smaller, lighter guitar amps are also available, which have less powerful amplifier units and as few as one speaker. Smaller guitar amps are easier to transport to gigs and sound recording sessions. Smaller amps are widely used in small venue shows, because players can obtain the tone they want without having to have an excessively loud volume. The smallest combo amplifiers, which are mainly used for individual practice and warm-up purposes, may have only a single 8 or 10-inch speaker. Some players use these small combo amplifiers for concert performances, though, because it is easier to create natural overdrive with these lower-powered amplifiers.

Powerful electric guitar amplifiers, such as Marshall amplifiers, are used in a range of louder, heavier genres of rock, including hard rock, heavy metal, and hardcore punk. These amplifiers can add an aggressive drive, intensity, and edge to the guitar sound with distortion effects, preamplification boost controls, and tone filters. This type of amplifier is available in a range of formats, ranging from small, self-contained combo amplifiers for rehearsal and warm-ups to heavy heads that are used with separate speaker cabinets—colloquially referred to as a stack. In contrast to the tweed-style amplifiers, which use speakers in an open-backed cabinet, companies such as Marshall tend to use 12" speakers in a closed-back cabinet. While many of the most expensive, high-end models use 1950s-style tube amplifiers, there are also many models that use transistor amplifiers, or a combination of the two technologies (e.g., a tube preamplifier with a transistor power amplifier).