This study is the first to employ rich data from both kindergarten cohorts of the nationally representative Early Childhood Longitudinal Study (ECLS 1998 and 2010) to compare whether the relationship between preschool participation and children's cognitive and behavioral outcomes--both at school entry and through the first years of elementary school--has changed over time. The 12 years between these two kindergarten cohorts were characterized by a heightened understanding of the importance of early childhood in the life course and relatedly, substantial increases in public preschool access. Across both cohorts, preschool participation is positively associated with cognitive outcomes at school entry, and although these associations are substantially smaller, they are still present at the end of first grade. We document negative associations between preschool participation and behavioral outcomes. Notably however, these relationships are far less pronounced for the recent cohort. Finally, across both kindergarten cohorts, we document a particularly large positive association between preschool participation and first grade cognitive outcomes for black children. Implications for both developmental science and policy are discussed. (author abstract)