Many facets of the biology of extinct animals will likely remain forever outside of the domain of empirical verification. However, with EPB it is possible to reach conclusions with varying degrees of certainty about anatomy, physiology and behavior in animals that we could never actually observe. In hypothesizing about the visual systems of extinct animals we are fortunate that much of retinal anatomy appears to have been conserved in many lineages over the last 300 to 400 million years of evolution. We are unfortunate in that we are not among the lineages in which this preservation occurred, so our own visual systems are not good models for the visual systems of most other animals. This review has focused upon the photoreceptors, the most peripheral part of the visual system. A similar review could be written to address comparisons of more central visual structures and how EPB could be applied for them. Interesting contrasts have already been drawn between cortical processing of visual signals in birds vs. mammals (Shimizu and Karten 1991). Ultimately, we can draw some strong conclusions about particular aspects of visual function in a number of extinct animals. However, we are currently scratching only the surface of the wealth of knowledge that we can in principle obtain from extant animals and apply to their extinct relatives.