Play is a quintessential activity of childhood, the cognitive implications of which have generated longstanding theoretical and empirical interest. This chapter begins with the difficult issue of how to define play, then considers the two classic play theories, those of Piaget and Vygotsky. Next it covers varieties of play, from sensorimotor to physical, and from exploratory to symbolic, including the developmental course of each type. The bulk of the chapter concerns six current focal issues in play and cognitive development: (1) pretend play and theory of mind, (2) how children make sense of symbolic substitutions in pretend play, (3) constraints on object substitutions in children's play, (4) how children discriminate pretend from real, (5) how babies are initiated into pretend play, and (6) whether pretend play improves developmental outcomes. The next sections of the chapter address group differences in play by gender, developmental differences (visual and hearing impairments, autism), and culture, and the final section addresses future directions in play research.