Guided by an ecological theoretical model, the authors used a series of multilevel models to examine associations among children's individual problem behavior, the classroom behavioral context, and school readiness outcomes for a cohort of low-income children (N = 3,861) enrolled in 229 urban Head Start classrooms. Associations were examined between early problem behavior (overactive and underactive behavior) at the child and classroom level and three dimensions of school readiness: cognitive skills, social engagement, and coordinated movement, assessed at the end of the preschool year. At the child level, younger children, boys, and underactive and overactive problem behavior were associated with lower school readiness skills. At the classroom level, classroom contexts early in the preschool year characterized by high levels of underactive behavior (e.g., social withdrawal among children) were uniquely and additively associated with lower school readiness skills. Contrary to hypotheses, there were no significant associations between classroom behavioral contexts characterized early in the preschool year by high levels of overactive behavior (e.g., socially disruptive or dysregulated behavior among children). Findings extend prior research in Head Start. Implications for early identification and intervention are discussed. (Contains 2 figures and 4 tables.)