Diabetes mellitus is a group of diseases characterized by high levels of blood glucose resulting from defects in insulin production, insulin action, or both. Diabetes is a serious health concern. The number of cases of diabetes mellitus is estimated to grow at a rate of 50% between 2000 and 2010. There are several types of diabetes: type 1 diabetes, type 2 diabetes, gestational diabetes, and other specific types of diabetes. Beta cell dysfunction plays a key role in the physiopathology of diabetes, even when insulin resistance, which is often present in several diabetes-related diseases, is considered among the causes of hyperglycemic type 2 diabetes. The prolonged hyperglycemia that is peculiar to all kind of diabetes has long term complications on several organs and systems. The diagnosis of diabetes is based on the evaluation of glucose plasma levels performed under fasting conditions or two hours after the oral ingestion of 75 grams of glucose. Currently, achieving and maintaining normal plasma levels of glucose are the aims of therapy for both type 1 and type 2 diabetes. Particularly, the therapy for type 1 diabetes is based on the administration of insulin, whereas that of type 2 diabetes changes over the time: diet and physical activity are the first treatments; oral hypoglycemic drugs are used as a second therapeutic step; and the administration of insulin is the last therapeutic option. The principal therapeutic innovation of the past ten years is represented by the tight and flexible control of glucose plasma level obtained by using the insulin analogues produced by recombinant DNA technology.