Prekindergarten programs are expanding rapidly, but to date, evidence on their effects is limited. Using rich data from Early Childhood Longitudinal Study, we estimate the effects of prekindergarten on children's school readiness. We find that prekindergarten increases reading and mathematics skills at school entry, but also increases behavioral problems. By the spring of first grade, effects on academic skills have largely dissipated, but the behavioral effects persist. Larger and longer lasting academic gains are found for disadvantaged children. Finally, we find some evidence that prekindergartens located in public schools do not have adverse effects on behavior problems.