Aggressive behavior is a common referral problem for individuals with developmental disabilities (DD), placing them at risk for institutionalization, social isolation, physical restraint, over-use of medication to treat behavior problems, exclusion from services, and becoming a victim of abuse. Aggression strains relationships between individuals being supported and their caregivers, whether professionals or family members. The treatment of aggression is persons with DD, with or without comorbid mental illness, remains a controversial area and changes in practice have been slow to come. The evidence related to pharmacotherapy and psychological treatment is, in general, either lacking or poor. This does not suggest that these treatments are necessarily ineffective but that there is not enough good quality evidence to support their usefulness. This review considers the prevalence and correlates of aggression, as well as possible causative factors. The relationship between mental illness, intellectual disability and aggression is explored. The psychopharmacological and psychological treatment literature is reviewed with implications for clinical care and future research.