Parents and policymakers are turning to preschools to better advance the school readiness and broader development of young children. Much remains unknown, including how preschool programs of varying quality affect diverse groups of children. This is particularly important as several California counties embark on ambitious and costly efforts to widen children's access to preschool, and the state's demographic make-up becomes increasingly heterogeneous. This brief focuses on the following questions: (1) Which families gain access to center-based programs? (2) What levels of developmental progress--in cognitive, social-emotional, and school readiness skills--do California children display as they enter kindergarten? How do these developmental levels vary across social class and ethnic groups? (3) Does exposure to center-based programs help to explain developmental gains in school-readiness skills displayed by children after accounting for the attributes of their parents and home practices? This study examines a representative sample of 2,314 California children who began kindergarten in 1998. Their families reflect the ethnic and social class diversity of California and reside across 327 zip codes. Earlier estimates of center enrollment rates by social-class and ethnic groups have been based on smaller samples or limited census data. (Contains 4 boxes, 2 tables, 6 figures, and 14 endnotes.)