The authors consider mature make-believe play a critical component of childhood that helps children develop new skills and learn to communicate. They argue that, although theoretical accounts of play have emphasized the importance of make-believe play for children to achieve social and academic competence, the absence of a reliable and valid measure of children’s mature make-believe play has hampered the evaluation of such claims. They seek to address this shortcoming with a review of the psychometric characteristics of existing assessments and with their findings from a new assessment using the Mature Play Observation Tool (MPOT), which they administered during a multiyear longitudinal study of twenty-six early-childhood classrooms. They found that children in classrooms scoring well on the MPOT better perform such skills as self-regulation, literacy, and numeracy. (author abstract)