Comorbidity is a term that has become increasingly common during the past 10 years or so, particularly in the field of psychiatry. The assumption that a patient has comorbid mental disorders is often helpful in under- standing his or her pathological condition and in formulating treatment policy and determining prognosis. The concept of the coexistence of two or more diseases is not at all new, conventional clinical psychiatry has tended to avoid it. Traditional psychiatric diagnostic criteria have been presented as a stratified structure of independent disease classifications, and various symptoms have been subsumed under a single psychiatric disease. In patients with depression, the comorbidity of anxiety disorder (panic disorder, generalized anxiety disorder), obsessive-compulsive disorder, drug dependence, alcohol dependence, post-traumatic stress disorder, or personality disorder is an issue. When depression is comorbid with another mental disorder, both disorders are reported to be severer, more likely to be refractory, and more likely to be associated with a poor prognosis. The concept of comorbidity expressed through the names of multiple diseases that satisfy diagnostic criteria is rational from one point of view, conforming as it does to certain operational diagnostic criteria, and is helpful in understanding a patient’s symptoms as well as in clarifying treatment and prognosis