An expression pedal is an important control found on many musical instruments including organs, electronic keyboards, and pedal steel guitar. The musician uses the pedal to control different aspects of the sound, commonly volume. Separate expression pedals can often be added to a guitar amplifier or effects unit and used to control many different aspects of the tone.

Because the source of power with a pipe organ and electronic organs is not generated by the organist, the volume of these instruments has no relationship with how hard its keys or pedals are struck; i.e., the organ produces the same volume whether the key or pedal is depressed gently or firmly. Moreover, the tone will remain constant in pitch, volume, and timbre until the key or pedal is lifted, at which point the sound stops. The expression pedal gives the organist control over the external source of power, and thus the volume, of the instrument, while leaving the user's hands free.

This system of dynamic control is completely distinct from the act of adding stops (in the case of pipe organs) or pulling more drawbars (in the case of organs and synthesizers). Furthermore, the expression pedal can influence the volume (and, to a lesser degree, the timbre) of a note while it is being played; unlike other instruments, in which the note typically decays after it is first sounded, the organist can increase the strength of a chord or note as it sounds by increasing pressure on the expression pedal.

An organ expression pedal is typically a large pedal, resembling an oversized automobile accelerator, either partially or fully recessed within the organ console and located either directly above or to the right of the organ's pedalboard. As the pedal is pressed forward with the toes, the volume of the sound is increased; as it is depressed with the heel, the volume is decreased. A stand-alone expression pedal used with electronic keyboards, amplifiers, and effects is usually a smaller pedal made of metal or plastic that can be placed on the floor and then connected to the device with an instrument cable.

Beginning in the nineteenth century, it became common for one or more divisions of pipes in a pipe organ to be enclosed in a wooden box, at least one side of which would consist of palettes that open and close in a manner similar to a Venetian blind. A mechanical (later electrical) mechanism connected the box to a pedal that the organist would use to open and close the shutters, adjusting the perceived loudness of the sound. When the box is shut (or closed), less sound is released into the venue. In American and British organs, the enclosed division is usually named the Swell, and the box surrounding the pipes is usually referred to as the swell box. Thus, the expression pedal is sometimes known as the swell pedal or swell shoe. Sometimes the swell pedal is referred to by its German name, schweller. Larger organs may have two or more expression pedals, allowing the volume of different divisions to be individually controlled.

No matter how well a swell box is designed, the sound of the pipes is altered by their enclosure. Even when the shutters are fully opened, the pipes do not speak as clearly into the room as they would if they were otherwise unenclosed. In some instances and applications, particularly for the performance of Romantic organ music, enclosure in an expression chamber can remove some of the shrillness of organ pipes. Romantic instruments frequently have more than one division (keyboard) under expression. On pipe organs, the expression pedal should not be confused with the crescendo pedal, which progressively adds stops as it is opened, building from piano to fortissimo.

Historically, the palettes were manipulated by a ratchet swell, a lever operated by the foot to the side of the console. The lever would fit into two or three different notches, which would lock the position of the lever, and therefore the shutters, in place. To change the position of the shutters, the lever would be kicked sideways to allow it to travel into a new position. The lever was weighted so that its default position was at the top, with the shutters closed. As the lever was lowered, the shutters would open.

The balanced swell pedal (as pictured above) was developed in the late nineteenth century so that the opening of the box could be fixed at any degree (not just the two or three options of the ratchet swell). This pedal is fitted above the center of the pedalboard. It usually rotates towards and away from the organist through a distance of about 90° from an almost vertical position ("shut") to a near horizontal position ("open"). Because the pedal is balanced, the organist does not need to hold it in position, and it will balance at any point in its travel. In addition, this location for most expression pedals, above the center of the pedalboard, is much more convenient for use by both feet if necessary (although it is usually operated with the right foot).