This series of two articles presents a model of play development for use in pediatric occupational therapy. Proposing to unify the theoretical approaches of sensory integration and occupational behavior, the model uses play as the unifying link between these two apparently different approaches. This first article reviews the major concepts of sensory integration and occupational behavior, in addition to discussing the differences and similarities between the two. General systems concepts are used as a framework upon which a model of play development is constructed. Three hierarchical levels of play are described—sensorimotor, constructive, and social—with each broken down into several developmental steps.