In this review we consider the nature and possible developmental functions of three forms of play: pretend, social, and locomotor. Play is defined in terms of dispositional and contextual criteria. First, the frequency of occurrence of each form of play across the period of childhood is documented. Developmental function of play is conceptualised in terms of immediate or deferred beneficial consequences. Four strategies for examining developmental function are reviewed: arguments from design, correlational analyses, experimental enrichment and deprivation, and cost-benefit analyses. Whereas most theories of play implicitly assume that during childhood it occurs frequently and has benefits deferred until adulthood, we suggest that some benefits of play are immediate.