This study employs data from both kindergarten cohorts of the Early Childhood Longitudinal Study (n ~ 12,450 in 1998; n ~ 11,000 in 2010) to assess whether associations between preschool participation and children's academic and behavioral outcomes--both at school entry (Mean age = 5.6 years in both cohorts) and through third grade--have changed over time. Findings are strikingly similar across these two, nationally representative, U.S. cohorts: preschool is positively associated with academic outcomes and negatively associated with behavioral outcomes both at school entry and as children progress through school. Heterogeneity is documented with respect to child and preschool characteristics. However, there is no evidence that associations between preschool and medium-term child outcomes differ by elementary school characteristics. [This is the online version of an article published in "Child Development."]