The world surrounding us is composed of objects. Object recognition is an important factor in our survival and functionality in the world. Through our senses, we learn the properties of objects and distinguish them from one another. We learn, in particular, that objects occupy "volume" and are bounded by "surfaces", the type of entities whose existence and properties are learned through a combination of senses. Eventually, our visual perception of the external world relies on our ability to distinguish various pieces of surfaces, to integrate collections of surfaces into parts of an object, and to fill any missing information by inference and other mechanisms that develop as part of our survival strategy. Thus, a theory of visual perception of surfaces is at the heart of any comprehensive theory of human perceptual organization. First studied by Gestalt psychologists early in this century, perceptual organization concerns how retinal images are structured, how the various regions and elements are perceived as being related to each other in terms of part-whole relations, as well as various geometrical relations. Among the most pervasive and difficult problems in vision science are the nature of perceptual organization and the mechanisms responsible for it.