By the study of a disorder in the general population, epidemiology attempts to accomplish a number of goals. The first one is to determine the prevalence rate and the distribution of a disorder. This implies the answering of questions such as what is the proportion of the population at risk that develops the disorder within a given time period? And also, who has the disorder, where and when? Epidemiology may further be very useful in looking at the boundaries of a disorder. Where does a disorder shade off into normality, and at which level of symptom severity, or of functional impairment, does an individual become a case? A third goal of epidemiology is to examine the mechanisms that explain how a particular individual got the disorder. What are the risk and protective factors that influence the presence of the disorder, and how do these factors interact and evolve over time? Finally, epidemiology is concerned with the need for and the use of services, including the assessment of the determinants of help-seeking behaviour, and the investigation of referral pathways. Up-to-date epidemiological data are therefore an essential prerequisite for rational mental health policies and appropriate planning of services. They will also provide necessary background information to clinicians and researchers in the design of preventive interventions and treatment programmes (Verhulst and Koot, 1992, 1995).