--- - |2+ The association between parental and peer attachment and youths’ bullying involvement is well documented. However, there is little research examining mechanisms linking the quality of relationships with parents and peers to bullying perpetration and victimization. The present study investigated the indirect effects of attachment to the mother, the father, and peers on bullying and victimization through children's negative automatic thoughts in a sample of 476 adolescents (10–17 years). Path analysis indicated that attachment to both parents as well as attachment to peers was indirectly related to both bullying and victimization through their relationship with adolescents’ negative automatic thoughts. When particular cognitive contents of negative automatic thoughts were examined, only hostile thoughts emerged as a mechanism explaining the association between poor relationships with parents and peers and bullying involvement as perpetrator, whereas poor attachment with each socialization agent had an indirect effect on bullying victimization via hostility and social threat thoughts. - Aggressive Behavior, Volume 44, Issue 6, Page 561-570, November/December 2018.