Stereoscopy (also called stereoscopics or 3-D imaging) refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. These two-dimensional images are then combined in the brain to give the perception of 3-D depth. Besides the technique of freeviewing, which must be learned by the viewer, three strategies have been used to mechanically present different images to each eye: have the viewer wear eyeglasses to combine separate images from two offset sources, have the viewer wear eyeglasses to filter offset images from a single source separated to each eye, or have the lightsource split the images directionally into the viewer's eyes (no glasses required; known as Autostereoscopy).

Stereoscopy creates the illusion of three-dimensional depth from images on a two-dimensional plane. Human vision uses several cues to determine relative depths in a perceived scene. Some of these cues are:

All the above cues, with the exception of the first two, are present in traditional two-dimensional images such as paintings, photographs, and television. Stereoscopy is the enhancement of the illusion of depth in a photograph, movie, or other two-dimensional image by presenting a slightly different image to each eye, and thereby adding the first of these cues (stereopsis) as well. It is important to note that since all points in the image focus at the same plane regardless of their depth in the original scene, the second cue, focus, is still not duplicated and therefore the illusion of depth is incomplete.