The first practical electronic instruments were stand-alone devices intended to be additions to the orchestra. One of the earliest of these instruments was the theremin, invented in the Soviet Union in 1919. It was first used in a Hollywood film by Miklós Rózsa to suggest mental illness in Spellbound (1948).

Rózsa went on to use it in The Lost Weekend, but it became most famous in science fiction scores such as The Thing (Dmitri Tiomkin, 1951) and The Day the Earth Stood Still (Bernard Herrmann, 1951). The theremin begins at 0:40 in this trailer:

The introduction of multitrack tape recording to Hollywood in the late 1940s allowed complex electronic devices to be recorded and overdubbed (layered) in a studio, apart from the orchestra, and then added to a film. Louis and Bebe Barron were pioneers in early studio electronic music, known for the first completely electronic score to Forbidden Planet. The score begins in this clip at 0:30:

In the 1960s, self-contained synthesizers became available and were used in movies such as A Clockwork Orange (Wendy Carlos, 1971) and The Omen (Jerry Goldsmith, 1976):

"Synth bands" and composers of the 1970s and 80s such as Tangerine Dream provided scores to non-science fiction films such as Chariots of Fire (Vangelis, 1981), Thief (Tangerine Dream, 1981), and Risky Business (Tangerine Dream, 1983). Here is an example from Blade Runner (Vangelis, 1982):