["Symbolic Interaction, EarlyView. ", "\nPlay is often dismissed as trivial, yet it is a fundamental and adaptive aspect of human and mammalian life. This paper develops a sociological theory of play, treating it as a total social fact that spans biological, psychological, and social dimensions. Drawing on classical and contemporary sociology, developmental psychology, neuroscience, and ethnography, I argue that play varies along two central dimensions: autotelic motivation—whether the activity is engaged in for its own intrinsic pleasure—and structural formalization—the degree to which rules, resources, and social arrangements shape the encounter. This framework accounts for a wide range of activities, from solitary and parallel play to games, sports, and collective cultural practices, while explaining variation in affective engagement, temporality, and relational dynamics. The paper further explores implications for studying childhood development, adult play, and highly ritualized or formalized activities, highlighting opportunities for integrating interdisciplinary research. By situating play as both serious and consequential, this work advances a positive sociology attentive to the pleasures and sociality that sustain human life.\n"]