This study investigated the development of relational and physical victimization in adolescent friendship networks over time. Using longitudinal social network analysis (SIENA) it was simultaneously tested whether similarity in victimization contributed to friendship formation (selection effects) and whether victimization of friends contributed to changes in victimization (influence effects). This was done for peer-reported relational and physical victimization separately in two middle schools (total N = 480; N = 220, 47% girls, in School 1; N = 260, 52% girls, in School 2) across three time points (Grades 6 through 8; M ages 11.5–13.5). Gender, ethnicity, and baseline aggression were controlled as individual predictors of victimization. Similarity in physical victimization predicted friendship formation, whereas physical victimization was not influenced by friends' victimization but rather by adolescents' own physical aggression. Peer influence effects existed for relational victimization, in that adolescents with victimized friends were more likely to increase in victimization over time as well, over and above the effect of adolescents' own relational aggression. These selection and influence effects were not further qualified by gender. The results suggested that both selection and influence processes as well as individual characteristics play a role in the co-evolution of friendships and victimization, but that these processes are specific for different types of victimization. Aggr. Behav. 39 :229-238, 2013. © 2013 Wiley Periodicals, Inc.