This chapter reviews the research on school composition and peer effects from three comparative perspectives--Catholic and public schools, single-sex and coeducational schools, and small and large schools. Most of the research is sociological, focuses on high schools, and draws on national samples. The chapter seeks to discern cumulative trends in this research as it has evolved over the past two decades. Catholic schools have consistently edged public schools in achievement, but whether they offer additional benefits to minority and economically disadvantaged students is inconclusive. Although earlier studies suggested a single-sex school advantage, more recent research finds no difference between the two school types. Student achievement is higher in smaller schools, specifically schools in the 600-900 range, and in smaller schools achievement is more equitably distributed. While most of the reviewed research examined compositional rather than peer effects, some studies have offered theoretical perspectives that implicate peer effects. Research on compositional and peer effects would be enhanced by further development of theory, education databases designed to investigate multilevel questions, broader application of multilevel statistical techniques, and a search for the mechanisms through which compositional and peer effects operate.