The two dominant approaches to immigrant assimilation, segmented assimilation and "new" assimilation theories, have been successful at reporting and analyzing between-group differences in assimilation patterns. However, studies of assimilation generally do not address differences at the individual level. Current theories of assimilation cannot answer the simple question that gets to the heart of individual-level differences: how do you account for siblings in the same family assimilating in different ways? The usual suspect variables – parents’ educational attainment, income and occupational status, nationality/religion, context of reception and experiences of discrimination for the group in the host country – cannot address this question because these factors are the same within a family. So if those variables ultimately do not explain assimilation at the individual level, what does? We argue that peers will significantly affect variations in cultural and economic assimilation. We examine data from The Integration of the European Second Generation (TIES) survey, looking specifically at second-generation Turks across Western Europe. We find peer effects substantially affect cultural and economic assimilation, effects not predicted by either dominant theory of immigrant assimilation. We suggest that future researchers of second-generation immigrant assimilation take more seriously the effects of past and present peers.