This study is a randomized control trial of full- versus half-day prekindergarten (pre-K). The full-day pre-K offer produced substantial, positive effects on children’s receptive vocabulary skills (0.275 standard deviations) by the endof pre-K. Full-day participants also outperformed their peers on teacher-reported measures of cognition, literacy, math, physical, and socio-emotional development. At kindergarten entry, children offered full day pre-K still out-performed peers on a widely used measure of basic literacy. The impact of the provision of full-day care on children’s home lives and families, as well as a deeper exploration into how time is being used in full vs.half-day settings, will also be reviewed.