Abstract: In adolescence a social transition occurs whereby youth shift from relying on parents to prioritizing their peer relationships for support and companionship. It is paradoxical that adolescent girls cultivate more supportive, higher quality friendships, but are twice as likely as boys to develop depression during this developmental stage. Mounting evidence indicates that the tendency to co-ruminate with peers, or rehash talk of ongoing problems and negative reactions, may account for this effect. In this talk I will discuss the developmental changes in adolescent peer relationships and review the mounting evidence that traditionally protective factors such as social support, inadvertently fosters depression risk when sought via maladaptive means. Finally I will present new findings supporting cognitive and affective mechanisms of how co-rumination increases adolescents’ vulnerability to depression.