Early childhood education programs [e.g., prekindergarten (pre-K)]-characterized by stimulating and supportive teacher-child interactions in enriched classroom settings-promote children's learning and school readiness (1- 3). But in the United States, most children, particularly those from low-income backgrounds, attend programs that may not be of sufficient quality to improve readiness for school success (4). States are adopting Quality Rating and Improvement Systems (QRISs) as a market-based approach for improving early education, but few states have evaluated the extent to which their QRIS relates to child outcomes. We studied the ability of several QRISs to distinguish among meaningful differences in quality that support learning. (author abstract)