This study examines whether extended participation in high-quality OST programs is positively associated with the development of literacy skills and social emotional learning skills. Funded by the Charles Stewart Mott Foundation, this research is taking place in Minnesota and Massachusetts. It is a partnership among four organizations: the American Institutes for Research (AIR); the National Institute on Out-of-School Time (NIOST); the Minnesota Department of Education; and the Massachusetts Department of Elementary and Secondary Education. Our goal is to follow youth who participate in these programs over the span of two consecutive years while enrolled in Grades 1 and 2 in Massachusetts and in Grades 4 and 5 in Minnesota. Findings will inform a better understanding of how extended participation in high-quality 21st CCLC programs is associated with growth and development of social-emotional learning skills and other essential skills and competencies measured over time, and development of key literacy skills.