The epidemiology of psychological disorders plays a significant role in the identification of the present state of mental health in the society and estimation of its requirements at any period of time. Based on worldwide epidemiological researches on mental disorders, the findings and instruments of 29 such researches carried out in Iran were reviewed. The review demonstrated that the studies varied in their tools, method and populations, which represented different socio-economic groups, and that the prevalence of mental disorders was therefore reported differently in different researches, ranging from B% to 53% among the various populations. The acceptable piont prevalence may therefore be estimated between 18% to 23% in the country, which is higher than that of western countries. One of the striking findings of the researches was that mental disorders are two to three times more common among women than among men. The review further examines some methodological problems of the epidemiology of mental disorders.