In this theoretical part, the term Comorbidity is defined. The necessity of (1) using international definitions of diseases; (2) specifying the time of Comorbidity (such as lifetime, 12-month or point Comorbidity); (3) not considering the coincidence of symptoms and syndromms as Comorbidity and (4) not acknowledging the combination of symptoms of the disease with the social manifestation of special groups (such as prisoners, suicides, war veterans, perpetrators of violence) as Comorbidity was pointed out. When studying the Comorbidity of mental and physical diseases, it is necessary to distinguish the Comorbidity and various manifestations of the same illness or various stages of the course of the same illness. The review of literature includes results of studies of Comorbidity in psychoses, depression, neuroses including anxiety, dependence, persons with suicidal behaviour, somatic diseases and patients of general practitioners. Also included are the results of Comorbidity in various groups and the influence of Comorbidity on health care costs.

        Key words: Comorbidity, mental disease, diagnosis.