The prevalence of and risk factors for aggression were examined in 1,380 children and adolescents with autism spectrum disorders (ASD). Prevalence was high, with parents reporting that 68% had demonstrated aggression to a caregiver and 49% to non-caregivers. Overall, aggression was not associated with clinician observed severity of ASD symptoms, intellectual functioning, gender, marital status, parental educational level, or aspects of communication. Individuals who are younger, come from a higher income family, have more parent reported social/communication problems, or engage in repetitive behaviors were more likely to demonstrate aggression. Given the significant impact of aggression on individual and family outcomes, it is hoped that this knowledge will inform more targeted intervention efforts.