Introduction There is substantial agreement among psychologists that “pure” disorders are relatively infrequent. The probability of fi nding a patient with only one disorder is low in many treatment settings, and quite often clinical practice is dominated by complex cases in which two or more diagnoses are applicable to the same patient. However, patients with multiple diagnoses are frequently excluded from research, thereby creating a gap between science and practice. In recent years, there appears to have been an increase in research interest in patients with comorbid disorders, particularly in the case in which a psychiatric diagnosis is accompanied by a substance use disorder. It is, for example, now widely recognized that patients with schizophrenia or bipolar disorders frequently often have alcohol or drug use disorders.