Among a nationally representative sample of 2336 Head Start children, patterns of school readiness were compared at the beginning and end of children's first preschool year, and predictors of stability and change across readiness profiles were examined. The present study documented that although the majority of children remain in a qualitatively similar school readiness profile across their first year in Head Start, 20% of children move to a qualitatively different profile over the school year, reflecting both improvements and declines in functioning. Child and family attributes (e.g., child age, ELL status, maternal education, and family structure), as well as contextual factors (e.g., teacher education and experience, parenting style, and parent involvement) were significant predictors of both profile stability and change. Given that we have little understanding about what factors practice or policy can manipulate to improve school readiness, these findings shed light on what we might do to promote school readiness and prevent declines in functioning over time. Thus, findings from this study provide a population- and pattern-based perspective of Head Start children's strengths and needs, relevant for informing both individual and systems level change in Head Start programs across the nation. (Contains 4 tables and 2 figures.)