Childhood psychiatric disorders are estimated to influence about 9 to 21% of relevant age group and interest in this disorders are increasing all over the world. The growing need to child and adolescent mental health leads the task of establishing proposals and policies in this field to become a priority for governments. The first step of such proposals should be determination of prevalence of child and adolescent mental disorders in that country. However, several major methodological problems make it hard to provide accurate prevalence estimates from epidemiological studies. Most common problems are within the fields of sampling, case definition, case ascertainment and data analyses. Such issues increases the costs of studies and hinder to reach large sample sizes. To minimize these problems, investigators have to be careful on choosing the appropriate methodology and diagnostic tools in their studies. Although there are many interviews and questionnaires for screening and diagnosing in child and adolescent psychiatry, only a few of them are suitable for epidemiological research. In parallel with the improvement in all fields of child and adolescent mental health in our country, some of the major screening and diagnosing tools used in prevalence studies in literature have already been translated and validated in Turkish. Most important of this tools for screening purposes are Child Behavior Checklist and Strengths and Difficulties Questionnaire and for diagnosing purposes are Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version and Development and Well-Being Assessment. The aims of this article are to review the methodological problems of epidemiologic studies in child and adolescent psychiatry and to briefly discuss suitable diagnostic tools for extended sampled epidemiologic studies in our country.