Students who attend preschool regularly are significantly more likely than chronically absent preschoolers, those who missed at least 10 percent of the school year, to be ready for kindergarten and to attend school regularly in later grades. The study, which followed 25,000 three- and four-year-olds served by Chicago Public Schools (CPS) school-based preschool programs, finds chronic absenteeism is rampant among preschoolers in Chicago. In 2011-2012, almost half of three-year-olds and more than one-third of four-year-olds were chronically absent. This report examines the extent of preschool absenteeism and the reasons preschool students are absent. It also examines the relationship between preschool absences and students' scores on measures of kindergarten readiness in math, letter recognition, and social-emotional development, as well as assessments of second-grade reading fluency. Ultimately, students who miss more preschool have lower kindergarten readiness scores, and students who are chronically absent in preschool are more likely to be chronically absent in kindergarten and have lower second grade reading scores. However, students who enter preschool with the weakest skills benefit the most from regular attendance.