Using an ecological systems conceptual framework proposed by Bronfenbrenner, research on the inclusion of preschool children with disabilities in programs with typically developing children was reviewed. Drawing mainly from studies conducted in the United States, research on child characteristics (biosystem), classroom practices (microsystem), family perspectives (mesosystem), social policy (exosystem), culture (macrosystem), and changes in variables across time (chronosystem) is described. Positive developmental and behavioral outcomes occur for children with and without disabilities in inclusive settings, although as a group, children with disabilities are not as socially integrated as their typically developing peers. Parent attitudes are generally positive although they voice some concerns about inclusion. Several social policy issues within the U.S. system (e.g., enforcement of standards, fiscal issues) serve as barriers to and facilitators of implementation of preschool inclusion, and cultural variables shape the nature of inclusive classrooms as well as family access to inclusive settings.