Chroma key is a technique for mixing two images or frames together, in which a color (or a small color range) from one image is removed (or made transparent), revealing another image behind it. This technique is also referred to as color keying, colour-separation overlay (CSO; primarily by the BBC), greenscreen, and bluescreen. It is commonly used for weather forecast broadcasts, wherein the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background.

The meteorologist stands in front of a bluescreen, and then different weather maps are added on those parts in the image where the color is blue. If the meteorologist himself wears blue clothes, his clothes will become replaced with the background video. This also works for greenscreens, since blue and green are considered the colors least like skin tone.

This technique is also used in the entertainment industry, the iconic theatre shots in Mystery Science Theater 3000, for example. Chroma Key technology is often preferred over having the actors or meteorologists stand in front of a giant television screen, due to its relatively low cost of production. Prior to the introduction of digital compositing, the process was a complex and time consuming one known as "travelling matte".

The blue screen and traveling matte method were developed in the 1930s at RKO Radio Pictures and other studios, and were used to create special effects for The Thief of Bagdad (1940). At RKO, Linwood Dunn used travelling matte to create "wipes" – where there were transitions like a windshield wiper in films such as Flying Down to Rio (1933).

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