What is a high-quality statewide data system? One characteristic is that it provides the information needed to address important questions about early intervention and early childhood special education. But what are those questions? What questions should data users, such as program directors, advocates, and policymakers, be asking? The Center for IDEA Early Childhood Data Systems (DaSy) compiled a set of Critical Questions that a quality state data system for early intervention (EI) or early childhood special education (ECSE) should provide the data to answer. The answers to these questions serve various functions, including supporting the state agency in effectively administering the program, meeting accountability requirements for EI and ECSE, and improving results for children and families through an examination of program features. (author abstract)