One of the situations in which the child spends much time is informal, outdoor play settings. Much of this time is spent in spontaneous play in neighborhoods, around the dwelling unit, and on designated playgrounds. Unstructured and spontaneous play is an important part of a child's development. It is through unstructured, child-initiated play that the child is most often free to explore, to test, and to learn from feedback from the environment. This paper: (1) reviews the current major theory of play and play environments; (2) reviews the current research on play environments; and (3) looks at some ideas for the planning and design of play environments based on that research. The basic questions asked are: (1) What is known from the scientific literature about the relations among the physical environment, children's play behavior, and subsequent development? and (2) How are the findings explained, and how are they used in applications to environmental policy, planning, and design? In investigating these questions, attention is paid to both designated playgrounds and neighborhood play spaces and comparisons made between the two. Eleven new implications for environmental policy and for the planning and design of play environments, based on the research, are offered. A 7-page bibliography, 6 tables, and 11 figures are included. (JMK)