The Thereminvox or Theremin is one of the earliest fully electronic musical instruments. Invented in 1919 by Russian Lev Sergeivitch Termen (later gallicized to Leon Theremin), the Thereminvox was an offshoot of government-sponsored research into proximity sensors. Consisting of a box with two radio antennas, the Theremin was unique in that it required no physical contact in order to produce music; instead, a performer could control both the pitch and volume of the sound simply by moving his or her hands in the air.

Based on the principle of heterodyning oscillators, the Thereminvox generates an audio signal by combining two different, but very high frequency radio signals. The capacitance of the human body close to the antennas causes pitch changes in the audio signal, in much the same way that a person moving about a room can affect television reception. By changing the position of the hands with regard to one antenna, a performer can control the pitch of the output signal. Similarly, the amplitude of the signal can be affected by altering the hand's proximity to the other antenna. A careful combination of movements can lead to surprisingly complex performances.

Leon Theremin's invention of the Thereminvox was followed closely by the outbreak of civil war in Russia. After rave reviews at Moscow electronics conferences, Theremin demonstrated the device to Bolshevik leader Vladimir Lenin personally. Lenin was so impressed with the device that he began taking lessons in playing it, commissioned 600 of the instruments for distribution throughout the Soviet Union, and sent Theremin on a trip around the world to demonstrate the latest Soviet technology and the invention of electronic music. After a lengthy tour of Europe, during which time he demonstrated his invention to packed houses, Theremin found his way to America, where he patented his invention in 1929. Subsequently, Theremin granted commercial production rights to RCA.

The Theremin was not an easy instrument to play. It required performers to remain absolutely still lest their body movements alter the pitch of the instrument, and maintaining pitch based on audio feedback alone proved difficult as well. Theremin's search for a virtuoso was answered by another Russian expatriate, Clara Reisenberg Rockmore (1911-1998), a talented violinist who saw great promise in Theremin's invention, particularly since her career as a violinist was cut short due to problems with her bowing arm. Rockmore came to be known as the definitive master of the Theremin, her precise fingering technique and her perfect pitch giving her extraordinary control over the difficult instrument. Other theremin virtuosos include Lucy Bigelow Rosen (1890-1968). Most of the strange-sounding theremin music in older science fiction movies was performed by Dr. Samuel J. Hoffman (1903-1967).

Although the RCA Thereminvox was not a commercial success, it fascinated audiences in America and abroad. Theremin continued his inventing, creating larger devices such as a platform-sized model intended to merge musical performance and dance. One day in 1938, Theremin was kidnapped from his New York apartment by Soviet agents, and forced to return to the USSR. Although rumors of his execution were widely circulated, Theremin was in fact put to work in a labor camp, where he designed the first "bug" (electronic listening device.)

The Theremin has enjoyed a renaissance from the 1950s to the present, being featured in films such as The Day the Earth Stood Still, Forbidden Planet, and Plan 9 From Outer Space, as well as popular music by The Bee Gees, Led Zeppelin, and (more recently), Portishead and The Flaming Lips. Danny Elfman made extensive use of the Theremin in the soundtrack to Tim Burton's spoof B-movie, Mars Attacks. Leon Theremin died in 1993, having seen his invention become popular all over the world. A distant relative of Theremin's, Lydia Kavina is today regarded as one of the masters of the instrument.

Although there are very few original RCA Theremins available, modern Theremins can be built from kits or blueprints, and even sport new features such as optical amplitude controls and MIDI converters.

Another electronic musical instrument, the Electro-Theremin, was used by The Beach Boys on Good Vibrations and I just wasn't made for these times, played by Paul Tanner. They used a modified device, constructed by Robert Moog, which is operated by mechanical controls, produces a musical tone similar in its timbre and portamento to that of the Theremin. This was played live by Mike Love.