Ever since the introduction of epidemiological methods into the field of child and adolescent psychiatry, numerous studies have been published in which epidemiology plays a major role. The question arises in which ways epidemiology contributes to our understanding of child and adolescent mental health and to our every day clinical practice. In this presentation I will give an overview of major accomplishments of epidemiological studies in our field. The most classical type of study is the prevalence study of which numerous studies have been carried out. Although progress has been made, a major obstacle to prevalence and other epidemiological studies is how to measure psychopathology. This is especially relevant in a world which is characterized by huge movements of children across countries and cultures. Derived from prevalence studies are studies aimed to answer questions regarding the continuity and discontinuity of child psychopathology, even across generations, and regarding historical trends. Much more recent are epidemiological studies that aim to answer questions regarding the etiology of child psychopathology.