Most accountability policies monitor Head Start quality at the center level by selecting a subset of classrooms within a center to represent quality. This study explores variation in classroom quality in Head Start and implications for accountability systems and children's well-being. We find that one third to one half of the variation in quality was due to differences between classrooms within center and that 37% of centers would receive different accountability decisions depending on which classrooms were selected. Average center-level quality was not related to children's development. However, differences in within-center classroom instructional quality were related to children's academic and social skills. Findings suggest that accountability systems miss important variation in classroom quality within centers, which may lead to inaccurate high-stakes decisions. (author abstract)