Periodontal disease indices are a must in epidemiologic studies, analytical epidemiology making special demands. The indices hitherto commonly used do not meet all demands; consequently, they can be applied only in certain studies, especially in the sense of descriptive epidemiology. For 15 years, the periodontal disease index according to Kötzschke has proved to be useful for large-scale correlation calculations, being well suited for making evident the effects of the most different factors. At the beginning of any experiment and investigation, exact considerations must be given to the aim and special objectives of an index.