A variable number of abnormalities combine to make up the syndrome known as “diabetes mellitus.” The definition of exactly what constitutes the diabetic state is somewhat arbitrary and semantic, and is often viewed differently by the experimentalist and the clinician. Hyperglycemia is the one condition common to all definitions of diabetes. The most common clinical type of diabetes mellitus and the diabetic states, which have been most thoroughly investigated experimentally, are caused by a primary lack or absence of insulin. It is perhaps a useful coincidence that in English the word diabetes is spelled like a plural. Certainly it ought to be considered as a collective noun, representing a class of very different conditions presenting the common characteristic of high blood glucose levels. Even in this regard, differences exist between hyperglycemias as regards their degree and their dependence upon the nature and the amount of the diet concerned. The chapter describes those diabetic states occurring clinically and produced experimentally which are not due primarily to a lack of insulin. Some of these hyperglycemic states may subsequently depend upon a lack of insulin for their persistence, but their geneses depend upon other factors.