To evaluate the adequacy and efficiency of preschool education, the RAND Corporation has undertaken the California Preschool Study to improve understanding of achievement gaps in the early elementary grades, the adequacy of preschool education currently given, and what efficiencies or additional resources might be brought to bear in early care and education. In California, there has been only limited information about the nature and quality of the early care and education (ECE) arrangements of preschool-age children — those who are one or two years away from kindergarten entry. What percentage of children participate in ECE programs at ages three and four? What is the quality of the programs in which they participate? How do access and quality vary for children of different racial or ethnic backgrounds or from low- and high-income families? This report answers these and other questions about preschool use and quality in California. The results of the study show that (1) use of center-based ECE is the norm for these children; (2) Latinos and socioeconomically disadvantaged children use it least; (3) these programs fall short on key quality benchmarks, especially those linked to early learning; (4) groups with lower levels of school readiness and later school achievement are least likely to participate in the programs that most promote school success; and (5) there is room for improvement in quality across the board and in raising participation for targeted groups.