This chapter covers a unique contextual factor that affects stereo viewing: perceptual constancy. Here, the general concept of perceptual constancy is used to represent a set of three perceptual constancies: size constancy, speed constancy, and depth constancy. These constancies represent stable and veridical perceptions of size, speed, and/or depth when objects in the natural world are seen. Such stable and verdical perceptions occur via a visual cue-integration process that combines information about retinal size, retinal speed, or retinal (binocular) disparity with visual estimates of egocentric viewing distance. Perceptual constancy is a ‘contextual’ factor because it involves the surrounding conditions that exist (e.g., distance cues) when someone views a stereo display.