This books adopts the phylogenetic perspective which the author developed to understand the evolutionary origins of play, and then applies the approach to current work in play, cognitive neuroscience, and development in humans. It reviews the history of the theoretical ideas as well as factual knowledge about animal play and reviews and evaluates current causal and functional approaches, often isolated from comparative and evolutionary analyses. It also contrasts the life histories and physiology of endotherms with those of reptiles and surveys the diversity of behaviour that can be linked to play within these groups.