This article introduces some of the terms used in psychiatric epidemiology when measuring the number of people in a community who have a disorder and the possible effects of prevention programs. Incidence is a count of the number of new cases of a disorder that occur within a defined time period. It can be expressed as either the proportion of people who can be expected to develop the disorder within that period (the cumulative incidence) or as the rate per person years (the incidence density). Various indices of prevalence refer to the total number of people who have the disorder at any one time, whereas indices of risk are used to define the probability of developing the disorder. The etiologic fraction is the proportion of cases due to a specific cause, and thus it reflects the maximum degree to which primary prevention programs can be effective.