Researchers, policymakers, practitioners, and parents have assumed a positive relationship between school attendance and academic success. And yet, among the vast body of empirical research examining how input factors relate to academic outcomes, few investigations have honed in on the precision of the relationship between individual attendance and student achievement. The purpose of this article is to provide insight into this relationship. Specifically, this study has evaluated the hypothesis that the number of days a student was present in school positively affected learning outcomes. To assess this, a unique empirical approach was taken in order to evaluate a comprehensive dataset of elementary and middle school students in the Philadelphia School District. Employing a fixed effects framework and instrumental variables strategy, this study provides evidence from a quasi-experimental design geared at estimating the causal impact of attendance on multiple measures of achievement, including GPA and standardized reading and math test performance. The results consistently indicate positive and statistically significant relationships between student attendance and academic achievement for both elementary and middle school students. (Contains 7 tables and 18 notes.)