Relational aggression includes behaviors intended to damage the social status and relationships of others. Recent work has demonstrated problematic outcomes for both aggressors and victims. These behaviors have been associated with psychopathology, including early borderline personality pathology. Despite the importance in better understanding these behaviors, numerous questions remain regarding the validity of different informants and methods for assessing relational aggression. In a community-based sample of 330 families evenly split by child gender (50.3% female), relational aggression data was assessed via multiple informants (mother, father, and self) and multiple methods (questionnaire and interview). The results suggest that mothers and fathers show higher agreement for girls’ relational aggression than for boys’ and that the presence of gender differences depends on the method of measurement. In addition, both mothers and fathers reports of relational aggression uniquely predicted internalizing behaviors, externalizing behaviors, and social problems, with fathers’ reports emerging as a particularly robust predictor. Relational aggression scores accounted for a greater amount of predicted variance in externalizing behaviors than in the other two domains. Results are discussed in the context of the utility of multiple informants and the conceptualization of relational aggression in a broader externalizing psychopathology framework.