Classmates can influence a student's academic achievement through immediate interactions (e.g., academic help, positive attitudes toward reading) or by sharing tangible or intangible family resources (books, stories of foreign travel). Multilevel analysis of 141,019 fourth-grade students' reading achievements in 33 countries showed that classmates' family factors (parent socioeconomic status [SES], home educational resources) were more strongly related to a student's reading achievement than were classmates' characteristics (parent ratings of past literacy skills, attitudes toward reading). However, these classmate links to reading achievement differed across students (e.g., high-SES classmates benefited high-SES students more than low-SES students). Also, links between classmates' past reading achievement and a student's current reading achievement were stronger in countries that were richer, were more collectivist, or avoided uncertainty less. These findings show how an ecological model of family and classmate microsystems, classmate family mesosystem, and country macrosystem can help provide a comprehensive account of children's academic achievement.