Treatments for people with the co-occurring disorders of mental illness and substance use (abuse or dependence) have been evolving and improving since the mid 1980s. During this period substance abuse treatment programs reported between 50 and 75% of the people they served also had a mental health problem. At the same time, mental health programs reported between 20 and 50% of the people they served had a co-occurring problem of substance use or abuse. The proposed solution is to integrate the two treatment systems so as to treat both disorders at the same time. During the 1990s integrated treatment was proposed and several models of integrated care were evaluated. The fallacy of these conceptualizations is that the authors of these models presume that the numerous treatment interventions from both treatment traditions are compatible. This is a critical assumption. What if the two are not compatible? Rather than integrate the two systems of mental health and addiction treatment in whole, I would argue, a model is needed that selects the best interventions from each field and discard the rest. Suggestions are presented here for a beginning inventory of best practice interventions that could be the foundation for effectively treating people with co-occurring disorders.