Cross-training professionals from mental health and addiction treatment systems can help further the goal of comprehensive treatment for clients with co-occurring mental health and substance use disorders (co-occurring disorders). Two such trainings brought together 122 professionals from mental health and addiction treatment fields. This evaluation study explored changes in treatment staffs' knowledge, beliefs, and cross-disciplinary collaboration over the course of the 22.5 hour training. Results suggest that participants benefited from exposure to content outside their respective discipline, and knowledge of co-occurring disorders and treatment for co-occurring disorders improved. Both groups of professionals endorsed similar beliefs about the nature and etiology of addiction over the course of the training. Differences emerged in participants' views about cross-disciplinary collaboration, with addiction treatment professionals reporting more agency collaboration. Participants favored aspects of the cross-training process, whereby they were able to work with professionals outside their discipline in both small and large group formats. (Contains 3 tables.)