This paper investigates the determinants of school friendship networks among adolescents, proposing a model of network formation and estimating it using a sample of about 10,000 secondary school students in four countries: England, Germany, the Netherlands and Sweden. We test the idea that networks arise according to homophily along many characteristics (gender, school achievement and ethnic and cultural backgrounds), and assess the relative importance of each factor. In addition to gender, we find that country of origin, generational status and religion predict friendship for foreign-born students. For individuals born in the survey country, ties depend on a broader set of factors, including socioeconomic status and school achievement. In sum, homophilic preferences go considerably beyond ethnicity. Multiculturalism, which gives prominence to ethnic backgrounds, risks emphasising the differences in that dimension at the expense of affinity in others. Keywords Friendship • Homophily • Immigration • Networks • Social cohesion JEL Classification D85 • J15 • Z13 Earlier versions of this paper were presented at the 2nd Florence-Konstanz workshop on Behavioural and Experimental Social Sciences (University of Florence) and the 2nd International CILS4EU User Conference (Utrecht University). We thank participants at these meetings, Paolo Brunori, Editor Klaus F. Zimmermann and two anonymous referees for their comments and suggestions. Aldo Rustichini thanks the National Science Foundation for the grant SES 1728056, "Rules based policies, and Intelligence in Strateic Behavior"