This study evaluates the effects of classroom peers on standardized testing achievement for all third- and fourth-grade students in the Philadelphia School District over 6 school years. With a comprehensive individual- and multilevel data set of all students matched to teachers, classrooms, and schools, two empirical strategies are employed. The first relies on the observable distribution of students among classrooms within a particular grade and school in any given year. As a test of robustness, a second strategy is employed, which depends on the idiosyncratic variation in classroom composition based on the random assignment of students entering the school at abnormal times during the academic year. Based on these empirical strategies, this study finds statistically significant classroom peer effects on standardized achievement, though the degree to which they impact performance differs based on peer characteristics.