This document discusses play behavior. Play is considered by many people to be useless; in fact harmful, as it may impede work. This attitude is related to historical and cultural patterns, particularly the work ethic of Western Civilization. Others consider play to be useful, adaptive activity. Evidence supporting this view is reviewed, including animal research, cross-cultural studies, and reports of play training producing generalized positive effects in therapy. This paper attempts to reconcile the differences of these views by looking at play as types of adaptation that maximize the capacity for variability. Play is considered a personally useful type of adaptation having to do with subjective reactions to experience rather than objective accommodation within it. Play responses mediate novelty, but do not guarantee its access to external cultural usefulness. The importance given to play in recent years is a manifestation of the need for novel thought because of the diversification of social roles in modern times. The structure of play is discussed; universally-determined and culturally-determined play are differentiated. Implications for education are considered, stressing the advantages of play training for creativity and adaptability. (DP)