A growing body of scientific evidence demonstrates that emotional development begins early in life and is closely connected with the emergence of cognitive, language and social skills. Early emotional development lays the foundation for later academic performance, mental health, and the capacity to form successful relationships. Despite this knowledge, most policies related to early childhood focus exclusively on cognitive development as it relates to school readiness, neglecting the importance of such capacities as the ability to regulate one's own emotions and behavior and to manage successful interactions with other people. As a result, many of our nation's policies, such as those that regulate child care provider training, availability of early childhood mental health services, and early identification and treatment of behavioral disorders, overlook emotional development as a focus of evaluation and intervention. This report presents an overview of the scientific research on how a child's capacity to regulate emotions develops in a complex interaction with his or her environment and ongoing cognitive, motor, and social development. It then discusses the implications of this research for policies affecting young children, their caregivers, and service providers. [This report was published by the National Scientific Council on the Developing Child.]