An electronic musical instrument is a musical instrument that produces its sounds using electronics. In contrast, the term electric instrument is used to mean instruments whose sound is produced mechanically, and only amplified or altered electronically - for example an electric guitar. Usually the instrument will have some way of controlling the sound, such as by adjusting the pitch, frequency, or duration of each note.

All electric and electronic musical instruments can be viewed as a subset of audio signal processing applications. Simple electronic musical instruments are sometimes called sound effects; the border between sound effects and actual musical instruments is often hazy.

French composer and engineer Edgard Varèse created a variety of compositions using electronic horns, whistles, and tape. Most notably, he wrote Poème Électronique for the Phillips pavilion at the Brussels World Fair in 1958.

Electronic musical instruments are now widely used in most styles of music. The development of new electronic musical instruments continues to be a highly active and interdisciplinary field of research. Specialized conferences, notably the International Conference on New interfaces for musical expression, have organized to report cutting edge work, as well as to provide a showcase for artists who perform or create music with new electronic music instruments.

Early electronic musical instruments
In the broadest sense, the very first electrified musical instrument was the Denis d’or, dating from 1753. It was followed by the Clavecin électrique by the Frenchman Jean-Baptiste de Laborde in 1761.

The first purely electronic musical instrument was the Telharmonium, built by Thaddeus Cahill in 1906. Employing electric generators and tonewheels to produce notes, it had a length of 60ft and a weight of 200 tons; because of a lack of suitable loudspeakers at that time, the music was distributed over the telephone network.

One of the many instruments constructed in the following decades was the Theremin, invented by Leon Theremin in 1917, which used a pair of heterodyned vacuum tube RF oscillators to make an audible tone that varied in pitch depending on the distance, and thus the capacitance, between the user and the instrument. This was followed in 1928 by the Ondes Martenot which had a keyboard as well as several auxiliary controllers.

The sound of the Ondes Martenot is used extensively in the Turangalîla-Symphonie and other works by Olivier Messiaen. However, these were not true synthesizers in the modern sense, as they were not configurable to produce a range of complex sounds by additive or subtractive synthesis, instead generating single pure tones with controllable pitch, amplitude and vibrato.

Ca. 1929 Friedrich Trautwein invented the Trautonium in Berlin. It was played with a resistor wire which has to be pressed against a metal plate. Oskar Sala was one of the first players and continued development until his death in 2002. Paul Hindemith wrote some compositions for it.

These early electronic instruments produced only pure tones and were frequently used to make avant garde music. In April 1935, Laurens Hammond introduced the Hammond tonewheel organ, which generated complex tones using an electro-mechanical principle derived from the design of the Telharmonium. Later Hammond used the Leslie speaker to achieve special modulation effects, and the resulting Hammond organ sound is still regarded as the benchmark for the “electric organ” sound. This sound can be simulated by many modern synthesizers and digital samplers.