Patients with both mental illness and substance abuse pose a major clinical challenge to mental health and substance abuse clinicians. The literature seems to support the hypothesis that mental illness and substance abuse occur together more frequently than chance would predict. Assessment and classification of these patients should be guided by clinicians' needs to make meaningful therapeutic judgments and to communicate effectively with each other in coordinating treatment. Different phases of treatment require different approaches to assessment and classification. In initial classification, the clinician should recognize the problem of dual diagnosis and resist premature assumptions about which diagnosis is primary. Long-term treatment and rehabilitation may require systematic evaluation of alternative clinical hypotheses about why a patient exhibits both disorders. This approach eventually may lead to better ways to assess, classify, and treat these difficult patients.