In this paper human play is examined from the perspective of its role in evolution. A model of social evolution is proposed to extend the model of biological evolution and to demonstrate the central role of play in the socialevolutionary process. Since in evolution play is a mechanism for its process, it demands species members who are playful. Thus, man is a player by virtue of species membership and the evolutionary pathway of the species. The implications of this deep-seated characteristic of playfulness are discussed in terms of human development. Finally, a set of principles is derived from the evolutionary-developmental characteristics of play. Application of these principles in clinical practice is demonstrated and discussed.