The limitations of two dimensional pictures as representations for reality are discussed. A review is done of the perceptual cues necessary to convey a sense of realism. These cues include, but are not limited to, binocular disparity, field of view, accommodation, vergence, and chromatic adaptation. Examples are given of how the physical characteristics of two dimensional pictures limit the use of these cues in computer graphic images. Techniques developed by artists and photographers to overcome some of these limitations are discussed.