Generally, the perceived deviance of friends was the most important predictor of externalizing problem behaviors. Besides its distinctive role in explaining variance in aggressive behavior, delinquent behavior, and self-esteem, it also interacted with frequency of contact with friends in the prediction of delinquent behavior; high contact with friends was only associated with more delinquent behavior when the friends were perceived as deviance; however, adolescent self-esteem was more strongly related to trust in friends. These findings confirm the study's hypothesis that perceived friends' deviance is more important for adolescent problem behavior; whereas, the quality of friendship relations is more determinative of adolescents' self-esteem. Only one interaction that involved ethnicity was significant; for Moroccan adolescents compared with Dutch adolescents, a high frequency of contact with friends was linked to a higher level of self-esteem. Gender differences were more pronounced than ethnic differences. Boys were found to have more contact with their friends, especially in the ethnic minority groups; whereas, girls reported a higher quality of friendships; girls also perceived their friends as less deviant than did boys. Boys had higher self-esteem, and girls reported a higher level of verbal aggression. The study sample consisted of 508 Dutch, Turkish, and Moroccan adolescents living in the Netherlands. They completed a battery of tests that measured variables related to friendship relations, aggressive behavior, delinquent behavior, and self-esteem. 6 tables, 2 figures, and 54 references