Previous experimental research has suggested that chimpanzees may understand some of the epitemological aspects of visual perception, such as how the perceptual act of seeing can have internal mental consequences for an individual's state of knowledge. Other research suggests that chimpanzees and other nonhuman primates may understand visual perception at a simpler level; that is, they may at least understand seeing as a mental event that subjectively anchors organisms to the external world. However, these results are ambiguous and are open to several interpretations. In this Monograph, we report the results of 15 studies that were conducted with chimpanzees and preschool children to explore their knowledge about visual perception.