It is now established that very significant numbers of people with severe mental illness abuse or depend on drugs and/or alcohol. This combination (Dual Diagnosis) leads to increased rates of violence and service use, a reduction in adherence to treatment regimes, an increase in susceptibility to human immunodeficiency virus (HIV) infection and is now found in in-patient populations. Because of their vulnerability to accidents and physical illnesses, dual diagnosis patients are found increasingly in accident and emergency departments, general medical wards and primary care settings. For this reason nurses and other health professionals working in general hospitals should be as aware as their mental health colleagues of the specific needs of this population. There are some excellent models of service organization and training for dealing with dual diagnoses populations in some parts of the USA. However, there is little such development in the UK. There are clear pathways to be followed, but the need for action is urgent.