Diabetes mellitus comprises of a group of heterogeneous disorders which have an increase in blood glucose concentrations in common. The current classifications for diabetes mellitus type 1-4 are described and the main features of type 1 and type 2 diabetes are compared to allow for better discrimination between these diabetes types. Furthermore, the criteria for the correct biochemical diagnosis during fasting and during oral glucose tolerance tests are summarized. These data form the basis of the recommendations of the Austrian Diabetes Association for the clinical practice in diabetes.