Many research findings about animal play apply to children's play, revealing structural and functional similarities with mammals in general and primates in particular. After an introduction to life-history theory, and before turning to humans, the author reviews research about the two mammals in which play has been studied the most extensively: laboratory rats and monkeys. He looks at the development of play, deprivation studies, gender segregation, and the functions of gender-differentiated forms of play. The gender segregation and sex differences in play parenting and rough-and-tumble play observed in many primates are also evident in children. Vigorous social-play benefits all children physically by developing strong bones and muscles, by promoting cardiovascular fitness, and by encouraging exercise habits that help prevent obesity. Unsupervised play also helps hone the skills of communication, perspective taking, and emotion regulation. For boys especially, rough-and-tumble play in early childhood provides a scaffold for learning emotion-regulation skills related to managing anger and aggression.