The first of two articles on play reviews the development of play in typically developing infants, toddlers, and young children, including Piaget's observations on the development of play; developmental play research following Piaget (research by Lunzer, Sinclair, Lezine, Lowe, Rosenblatt, Uzgiris and Hunt, Fenson and others, Watson and Fischer, Ungerer and others, and McCune-Nicolich). (Contains references.) (DB)