Growing interest in publicly funded programs for young children has drawn attention to whether and how Head Start and other early childhood programs should be asked to prove their worth. Congress asked the National Research Council for guidance on how to identify important outcomes for children from birth to age 5 and how best to assess them in preschools, child care, and other early childhood programs. The Research Council's new report concludes that well-planned assessments can inform teaching and efforts to improve programs and can contribute to better outcomes for children, but poor assessments or misuse of the results can harm both children and programs. The report offers principles to guide the design, implementation, and use of assessments in early childhood settings.