Different measures may be used to describe how often disease (or another health event) occurs in a population. Incidence expresses the development of new cases and is mostly used against the background of prevention, to assess disease etiology or to determine the risk factors of disease. Depending on the specific study question, incidence may be reported as risk or as incidence rate. This paper discusses that it is preferable to use incidence rate in case of a dynamic population or in cases where the observation period is sufficiently long for competing risks or loss to follow-up to play a significant role. Prevalence is the number of existing cases, which is affected by both the number of incident cases and the length of disease time. It reflects the burden of disease on a population that may, among others, be measured in terms of costs or morbidity. Knowledge about this burden can be used for the planning of health-care facilities. This paper discusses the different measures of disease occurrence using a number of examples taken from the nephrology literature.