For the past 25 years, forensic psychiatry has been concerned with violent offenders with psychosis and/or personality disorder. If dual diagnosis or comorbidity meant anything to a forensic psychiatrist, it would be the ‘typical’ forensic case – an individual with schizophrenia and a premorbid dissocial personality who had been arrested for a violent crime. In this article I use comorbidity to describe the co-occurrence of two or more conditions (here a psychiatric disorder and health problems arising from substance misuse) rather than dual diagnosis. In fact, many violent offenders have multiple diagnoses. Williams & Cohen (2000) argue that dual diagnosis suggests a closer relationship, perhaps including cause and effect, and is a subset of comorbidity.