This article describes the importance of including a diagnostic play session as a major component of a comprehensive psychiatric assessment of young children. It is through observing a young child's play that a clinician can gather a wealth of information about a child's developmental level in the cognitive, social, language, and motor domains and some insight into the development of a child's personality and his or her inner world. This data, combined with the results of developmental testing, school observations, and parent interviews, can be important in making formulations and diagnoses and planning interventions.