A review of the extant literature indicates that a relationship exists between Borderline Personality Disorder (BPD) and externalized aggression. BPD is related to aggression in inpatients, criminal behavior, and violence. One study has noted a relationship between BPD and relational aggression. Despite knowledge of these associations, the mechanisms that are responsible for the relationship between BPD and aggressive behaviors are not known. Some researchers theorize that BPD is characterized by emotional hyperreactivity, particularly to fear-inducing stimuli. The goal of the proposed study is to examine three possible factors that may affect the relationship between BPD and aggression. They include comorbid diagnoses of Antisocial Personality Disorder (ASPD), gender, and psychophysiological arousal. Two separate articles were prepared to explore these variables. The study focused on the association between symptoms of BPD and relational, as well as physical aggression. The first study focused on the roles of ASPD and gender in the relationship between BPD symptoms and aggression. Results of the first study indicated that the number of symptoms of BPD was positively related to scores on self-report measures of both physical and relational aggression. The results for the relationship between symptoms of BPD and self-reports of relational aggression remained significant after controlling for symptoms of ASPD, but the relationship between symptoms of BPD and self-reported physical aggression did not. Gender did not moderate the relationship between symptoms of BPD and either type of self-reported aggression. The second study focused on variables of psychophysiological arousal. It was hypothesized that fear-based arousal would mediate the relationship between BPD symptoms and aggression, while general psychophysiological arousal would moderate the relationship. Symptoms of BPD were not associated with psychophysiological arousal to fear inducing stimuli; therefore, it did not mediate the relationship between symptoms of BPD and self-reports of aggression. General psychophysiological reactivity did moderate the association for self-reports of relational, but not physical aggression. Greater arousal was associated with a stronger relationship between BPD symptoms and self-reports of aggression. Implications for future research, assessment, and treatment were discussed.