Many argue that the composition of a school or classroom-that is, the characteristics of the students themselves-affect the educational attainment of an individual student. This influence of the students in a classroom is often referred to as a peer effect. There have been few systematic studies that empirically examine the peer effect in the educational process. In this research, we examine the peer effect with a unique data set that includes individual student achievement scores and comprehensive characteristics of the students' families, teachers, other school characteristics, and peers for five countries. The data allow an examination of peer effects in both private and public schools in all countries. Our analysis indicates that peer effects are a significant determinant of educational achievement; the effects of peers appear to be greater for low-ability students than for high-ability students. The finding is robust across countries but not robust across school type. © 2000 by the Association for Public Policy Analysis and Management.