Diabetes mellitus is a disorder of metabolism, most prominently carbohydrate metabolism. It is a disease characterized by persistent hyperglycemia (high glucose blood sugar). It is a metabolic disease that requires medical diagnosis, treatment and lifestyle changes. The World Health Organization recognizes three main forms of diabetes: type 1, type 2 and gestational diabetes (or type 3, occurring during pregnancy), although these three "types" of diabetes are more accurately considered patterns of pancreatic failure rather than single diseases. Type 1 is generally due to autoimmune destruction of the insulin-producing cells, while type 2 and gestational diabetes are due to insulin resistance by tissues. Type 2 may progress to destruction of the insulin-producing cells of the pancreas, but is still considered Type 2, even though insulin administration may be required.

Since the first therapeutic use of insulin (1921) diabetes has been a treatable but chronic condition, and the main risks to health are its characteristic long-term complications. These include cardiovascular disease (doubled risk), chronic renal failure (it is the main cause for dialysis in developed world adults), retinal damage which can lead to blindness and is the most significant cause of adult blindness in the non-elderly in the developed world, nerve damage, erectile dysfunction (impotence) and gangrene with risk of amputation of toes, feet, and even legs.