Visual knowledge is an enormously important part of our total knowledge. The psychological study of learning and knowledge has focused almost exclusively on verbal materials. Today, the advance of technology is making the use of visual communication increasingly feasible and popular. However, this enthusiasm involves the illusion that visual communication is transparent and overcomes problems of comprehension. The effort to understand how visual media can be used effectively in educational communication returns to the most basic and general questions about how visual information is coded and processed. Everything that is learned in the history of research on vision is relevant to the understanding of effective visual representation. Some of the gaps in knowledge are made more evident when this problem is analyzed. As yet, for example, there is not much to say about the kinds of visual comparison processes or perceptions of relationships that are involved in using graphs to answer questions about the information they represent. A kind of deep unity is emerging in what had been some very disparate areas of investigation--abstract pattern perception, graph reading, imagery, spatial ability--a unity that arises from the common involvement of a perceptual system that evolved to serve particular functions. Hypothetical though it may be, this source of unity can guide researchers in more fruitful and systematic investigations than has been seen in the past. Visual knowledge builds on a very powerful perceptual apparatus, but it is knowledge nevertheless, and it continues to grow and develop throughout life. (Twenty-five references and 18 photocopies of slides are attached.) (RAE)