National policy affects local practice in a variety of ways and through a variety of mechanisms. In this article, the authors examine what has been learned from Individuals With Disabilities Education Act's (IDEA) two early childhood (EC) programs about the power and limitations of policy as a lever to improve the lives of young children. Ecological theory provides a useful framework for understanding how IDEA directly and indirectly influences the provision of services to young children with disabilities and their families. Experience implementing Part C and Part B Preschool for 25 years not only confirms the power of the law's vision but also reveals some barriers to effective nationwide implementation. Implementation issues in four areas are discussed: access to services, the quality of services, cost and funding, and outcomes. The current move to build coordinated and integrated EC systems in states presents new opportunities and new challenges in these areas for IDEA's early childhood programs. (Contains 1 figure.)