Attendance in U.S. preschools has risen substantially in recent decades, but gaps in enrollment between children from advantaged and disadvantaged families remain. Using data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-1999, we analyze the effect of participation in child care and early education on children's school readiness as measured by early reading and math skills in kindergarten and first grade. We find that children who attended a center or school-based preschool program in the year before school entry perform better on assessments of reading and math skills upon beginning kindergarten, after controlling for a host of family background and other factors that might be associated with selection into early education programs and relatively high academic skills. This advantage persists when children's skills are measured in the spring of kindergarten and first grade, and children who attended early education programs are also less likely to be retained in kindergarten. In most instances, the effects are largest for disadvantaged groups, raising the possibility that policies promoting preschool enrollment of children from disadvantaged families might help to narrow the school readiness gap. Data from parent, teacher, and administrator surveys as well as a faculty checklist completed by the study's field supervisors are described and outlined in detail in the Appendix. (Contains 8 tables and 8 footnotes.)