Treatment of patients with concurrent mental illness and substance abuse represents a challenge to the traditional treatment systems. This article gives: 1) an introduction of the concept and frequency of dual diagnosis (DD), 2) a presentation and discussion of the latest guidelines on DD treatment, 3) status on the current situation in the DD field in Denmark, and 4) potentials for future research. The article is based on systematic examination of evidence-based research and popularized latest guidelines on DD treatment. Methodologically, both treatment and research is challenged by the diversity in DD combinations. Although integrated treatment with the inclusion of cognitive-behavioural therapy, motivational interviewing and family intervention in DD treatment show promising results, it remains to establish which treatment programme is the most qualified in improving mental health and reducing substance use. A future priority is the development of DD treatment that targets specific co-morbid combinations and treatment needs.