"Play" is a word used commonly to refer to children's preferred activities and to some adult activities, and it is often said that play promotes learning. But what is play exactly, and what and how do children learn through play? This essay begins with a description of an evolutionary, practice theory of play by German philosopher and naturalist, Karl Groos, followed by a system of categorizing play according to the kinds of skills most obviously practiced: physical/locomotor play, constructive play, language play, fantasy play, social play, and play with formal rules. Play is then defined as activity that (1) is self-chosen and self-directed, (2) is motivated by means more than ends, (3) is guided by mental rules, and (4) includes a strong element of imagination. These characteristics are elaborated upon to show how each contributes to play's developmental value. Two final sections describe the special developmental value of age-mixed play and deleterious changes in children's well-being that have accompanied the decline of play in recent decades.