Comorbidity can be described broadly as the presence of more than one disorder in a person in a defined period of time (Wittchen & Essau, 1993). Stimulated by the introduction of explicit diagnostic criteria and operationalised diagnoses in the Diagnostic and Statistical Manual of Mental Disorders (DSM–III; APA, 1980) and the Diagnostic Criteria for Research in ICD–10 (WHO, 1991), numerous studies in the 1980s and early 1990s, have assessed the extent, the nature, and more recently, the implications of comorbidity for a better understanding of mental disorders. Most studies investigated the association of anxiety and mood disorders, but increasingly there are also studies looking into the association of mood disorders with other disorders (such as somatoform and substance use disorders (Wittchen et al, 1993, 1996)) as well as with somatic conditions (axis II) and personality disorders (axis III).