Past research showed a relationship between substance abuse and aggression and past trauma and aggression. The nature of the relationships between substance use, trauma, and aggression is inconclusive. The current research hypothesized greater aggression among those with a history of substance abuse and trauma compared with those without such a history and an additive relationship between substance abuse and trauma on aggression. Participants were 615 individuals in a substance abuse treatment program for individuals under criminal justice supervision. Data were collected from face-to-face interviews and self-report measures. Univariate and multivariate analyses assessed the relationships among substance use, trauma, and aggression. Participants with a history of trauma and regular substance use reported the highest rates of homicidal ideation, problem behaviors, and person offenses. Participants in this group also reported greater desire for help managing their stress and tension, dealing with problems in their intimate relationships, developing healthier relationships in general, and learning prosocial approaches to express their feelings. Substance abusing participants who experienced trauma reported more externalizing behaviors and a greater desire for coping- and social-skills training than participants who abused substances but did not report a history of trauma. This suggests that participants in substance abuse treatment programs may have improved outcomes with the addition of components to address these issues.