Before discussing the practical aspects of clinical diagnostic assessments for patients with co-occurring disorders, a discussion of the etiological theories of these disorders, as well as any evidence that supports a particular model, is in order. Four general models have been used: common factor models, two etiological diagnostic models (secondary SUD and secondary psychopathology models), and bidirectional models (Mueser et al. 1998). Although some of the models have more data supporting them than others, no single model has clear explanatory power by itself.