Consistent school attendance is a foundation of student learning. While missing one or two days of school each year is not likely to have serious consequences, chronic absenteeism is related to significantly lower outcomes for students. Research shows that chronic absenteeism undermines the academic performance of adolescents. And new research suggests that absenteeism is not only a problem among adolescents but also is a significant problem among very young students: 11 percent of kindergarteners across the nation are chronically absent. Kindergarten students who miss more school learn less during the school year. While policymakers and others might be tempted to assume that attendance similarly affects students in kindergarten and preschool, there is very little research on attendance in the preschool years and whether it matters for learning outcomes. Given that many children start their formal schooling in preschool and because the promise of preschool is to prepare children for kindergarten, it is critical to know whether absenteeism undermines that promise. To address this gap in research, the University of Chicago Consortium on Chicago School Research (UChicago CCSR) partnered with the Office of Early Childhood Education at the Chicago Public Schools (CPS) in 2011 to study absenteeism among CPS preschool students. This report outlines key findings from this study. It describes the extent of absenteeism among preschool students and compares it with absenteeism among students in kindergarten through third grade; examines the relationship between preschool absenteeism and learning outcomes, both during preschool and in second grade; and explores reasons why preschool students miss school. (author abstract)