Child care is increasingly viewed as an opportunity to enhance children's development and school readiness, with prekindergarten programs and early intervention programs targeting children at different moments of development. Results of existing research are mixed, and although many children experience different child care arrangements at different ages, little is known about the joint influence of early and later child care experiences. Using Early Childhood Longitudinal Study-Kindergarten Cohort data, the authors estimate a series of regression models, examining the unique and additive contributions of initial child care experiences and prekindergarten experiences on children's school readiness. The authors find that early use of nonparental care is associated with negative sociobehavioral outcomes; prekindergarten center-based and Head Start care add to this negative association. Early participation in center-based care is associated with enhanced reading and math scores; those relationships are fully mediated by prekindergarten center-based care participation. Implications for policy, practice, and research are discussed. (Contains 3 tables.)