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. Despite rising achievement levels in recent years, a substantial percentage of second- and third-graders do not meet state education standards in English-language arts and mathematics. Some groups of students are falling short by larger margins than others. English learners and students whose parents did not graduate from high school have the highest proportion who fall short of proficiency in second and third grade. Percentages of black, Hispanic, and economically disadvantaged students falling short of proficiency in the same grades are also high. Measures of student performance in kindergarten and first grade show similar patterns of who is ahead and who is behind. Preschool appears to be a promising strategy for narrowing achievement differences. The size of the achievement gaps that currently exist and the strength of the evidence of favorable education benefits from well-designed preschool programs make a solid case for considering preschool as a component of a multi-pronged strategy for closing achievement gaps in California.