Over the last 20 years in North America, the prevalence of comorbid mental health and drug addiction problems has aroused increasing interest, especially with regards to treatment. As the networks of drug addiction and mental health services have developed in parallel, service integration represents an important issue in the best practices of co-occuring disorders treatment. Since treatment programs have been developed particularly for patients suffering from severe and persistent mental health disorders, who are least able to negotiate the services they require, it is the “one provider” model of service integration that has been adopted. The relevance of this model does not necessarily apply to other clienteles, however. In the domain of treatment, as in prevention, current research indicates that multi-dimensional intervention is the most effective. In this context, it is suggested to pursue research which examines the possible range of expertise developed at the treatment level and to explore the possible links with prevention programs, especially for clients at risk, in order to develop more effective approaches.