Growing interest in patients with dual diagnosis (DD) results from the rising number of such cases and difficulty in their treatment. A process of drug use "normalization", i.e. regarding drugs as something "normal" in the youth culture, has been observed for over a decade now. Psychoactive properties of drugs often precipitate or are superimposed on symptoms of mental illness. Drug abuse/dependence is most often concurrent with the following mental diseases/disorders: depressive episodes, neurotic or personality disorders, eating disorders, schizophreniform psychoses, schizophrenia and organic syndromes. As regards DD patients, particular attention is paid to those with psychotic symptoms. Whether hallucinogens may trigger an endogenous mental illness, e.g. schizophrenia, remains an open question. Clinical observations confirm that in comparison to those suffering from a single disorder, DD patients receive treatment for a shorter period, are hospitalized more often, more frequently manifest aggressive and suicidal behaviors, and their treatment outcome is poorer. On the grounds of clinical experience the following general principles of treatment and management of DD patients have been generally accepted: simultaneous pharmacological treatment of substance dependence and mental disorder(s), relapse prevention, predominance of individual psychological therapy, slow but systematic implementation of the treatment plan, continuity of care.