Diabetes mellitus is a medical disorder characterized by persistent variable hyperglycemia (high blood sugar levels), resulting either from inadequate secretion of the hormone insulin, an inadequate response by the body's cells to insulin, or a combination of these factors.

The most common forms of diabetes are type 1, type 2 and gestational diabetes. Type 1 (10% of cases) is due to destruction of the islets of Langerhans, which produce insulin; it can only be treated with insulin injections. In type 2 (90% of cases) the main problem is decreased sensitivity of the body's cells to insulin, known as insulin resistance, although insulin production is often affected in the later stages; as well as diet and exercise, treatment usually is with tablets, or eventually with insulin injections. Gestational diabetes may develop during pregnancy and is similar to type 2 in its mechanism; it may affect fetal health and 40% of women with gestational diabetes develop type 2 diabetes later on in life.

Since the discovery 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), to gangrene with risk of amputation of toes, feet, and even legs.