Aggression in people with a dual diagnosis is very challenging for service providers in community and clinical settings. The purpose of this paper is to review concerns expressed by community service providers and hospital staff regarding aggression displayed by the adults with developmental disabilities that they support. Findings are based on input received during province-wide focus groups on dual diagnosis in Ontario's psychiatric hospitals. The two most significant themes that emerged regarding aggression were: client aggression frequently leads to hospitalization, and, once in hospital, aggression prevents clients from being transferred back to community-based services. This paper concludes with participants' recommendations for the potential role of developmental services in dealing with aggression.