The present study examines both the unique and the combined role of best friends' delinquency and perceived friendship quality in the development of adolescent delinquency. Questionnaire data were gathered from 435 Dutch adolescent best friends (mean age at first wave = 12.97) over a period of 5 years with annual assessments. Results showed that mean levels of delinquency and perceived friendship quality increased over time. Adolescent best friends were highly similar in both mean levels and changes in delinquency over time. For boys, similarity in mean level delinquency between best friends was higher than for girls. In addition, only for boys, friends' delinquency is associated with increases in adolescent delinquency over time, and adolescents' delinquency is associated with increases in friends' delinquency over time. No bidirectional longitudinal associations were found between perceived friendship quality and adolescent delinquency. No interaction effects between friendship quality and friends' delinquency on adolescent delinquency were found. Thus, findings were more in support of the differential association theory than of the social control theory.