Publisher Summary This chapter discusses the study of living of phylogenetically ancient organisms such as insects, fish, amphibians, reptiles, and primitive mammals that give clues to the function of sleep. This phylogenetic approach toward investigating the origins of sleep has not been without controversy, and there is disagreement in the literature about the presence or absence of sleep in nonmammals. There is a general agreement that most nonmammalian organisms exhibit behavioral sleep or rest. However, the electrophysiological signs of sleep in nonmammalian organisms may be very different from that of mammals. This observation has led to the conclusion that nonmammalian species do not sleep because mammalian electrophysiological correlates of sleep are not present. Simple behavioral observations as well as correlations between surface brain activity and behavior in unstudied, interesting nonmammalian organisms are no longer adequate to advance the understanding of the function of sleep from a phylogenetic perspective. The same rigorous and innovative neurophysiological and molecular methodologies that have been applied to the study of mammalian sleep must also be applied to nonmammalian species.