A significant number of offenders have both mental health and substance use problems. Failure to identify and treat these complex needs can lead to poor outcomes, such as relapse and re-offending. Many staff working in the criminal justice sector lack access to appropriate training in this area - a point identified in the Bradley Report (Department of Health, 2009a). A training project was established to develop and evaluate the feasibility of training in dual diagnosis interventions for staff working in the criminal justice system. This was part of a Skills for Health national demonstration site. The course was developed based on Skills for Health learning design principles, using a competence-based approach. The training was delivered to 58 staff from a wide range of services. The feedback was generally positive; however, a number of issues related to the mental health training needs of people from a substance use background were identified. This paper will describe the process of development of the training and the implications of the feedback for the future development of such workforce development initiatives.