In Chapter 7, the reader is invited to understand that the capacity to see the world in three dimensions, that is, of perceiving depth, is based on the availability and contribution of several cues. The main cues for perceiving depth could be based on binocular viewing (e.g., retinal disparity), or monocular viewing (e.g., occlusion or perspective). A mechanism called perceptual constancy, by which almost everything seems to maintain basic characteristics, is then explained. An outstanding question is whether or not size constancy requires distance to be taken into account. An old experimental attempt for responding to the question, that reported by Holway and Boring, is described. The last part of the chapter is dedicated to special cases, optical illusions, where an observer is misled by the cues contained in a visual scene. Several illusions are described, emphasizing the case of the moon illusion and the interpretation of it proposed by Kaufman and Rock.