This study examined romantic partner selection and socialization of substance use (cigarettes, alcohol) and behavior problems among a sample of 78 young adolescents (6th-8th graders) over eleven months. Adolescent and romantic partner behaviors were assessed before and after relationships were initiated via school records and self-report. Most selection and socialization effects were apparent for the eighth grade adolescents (at Time 1). Prior to their relationship, eighth graders and romantic partners were alike on alcohol use. In contrast, romantic socialization effects emerged for eighth graders' cigarette use and behavior problems. The nature of the partner socialization effects depended on the combination of adolescents' and partners' pre-relationship behaviors. Eighth graders who dated partners with fewer problems showed the greatest instability in their behavior problems and partner behavior predicted greater decreased in problem behaviors among adolescents with more problems. The implications of these findings are discussed within the broader context of adolescent peer relationships. (Contains 4 figures and 4 tables.)