Med., 1983, 141, Suppl., 41-54 — Systematic studies of diabetes prevalence and incidence are now indicating the extent and impact of the disease and its complications. A major handicap in establishing a true global picture has been the lack of uniformity in defining and classifying diabetes, and an additional limiting factor has been the grouping of differernt types of diabetes in published prevalence data. It appears the prevalence of insulin dependent (Type 1) and non-insulin dependent (Type 2) diabetes varies between and within ethnic groups, e.g rural versus urban dwellers, thus making it difficult to compare data from different countries. However, this fact provides unique opportunities to study the relative importance of genetic and environmental factors in the aetiology of diabetes and its micro- and macrovascular complications.Probably the most urgent needs have been the standardization of classification and criteria, and of epidemiological methodology.This paper will cover the epidemiology of diabetes in terms of the new WHO classification :a. Insulin dependent diabetes mellitus=Type 1.b. Non-insulin dependent diabetes mellitus=Type 2.c. Other types, e.g. pancreatic or tropical malnutrition, endocrine, drug induced, etc.The main forms of diabetes seen in developed countries are Types 1 and 2. They appear to be two distinct clinical entities, but the concept of genetic-environ-mental interaction in their causation is probably equally applicable. In develop-ing countries, Type 2 diabetes appears to be the most common form, but tropical malnutrition diabetes also occurs in many regions of the globe — IDDM; NIDDM; epidemiology.