Diabetes mellitus is a group of metabolic disorders having features of hyperglycemia. The prevalence of diabetes is increasing sharply in the developing countries because of more sedentary lifestyles. India and China have the largest prevalence of diabetics. Normally, the blood glucose levels are maintained in a very narrow range of 70 to 120 mg/dL. Type 1 Diabetes and Type 2 Diabetes are two type of diabetes mellitus. Incidence of type 1 diabetes is greater in twins of affected individuals than in the general population. Type 2 diabetes is a multifactorial disease. Type 2 diabetes has a concordance rate of 35–60% in monozygotic twins compared with 17–30% in dizygotic twins. Hyperglycemia (glucotoxicity, insulin resistance, and obesity are the pathogenesis of the complications of diabetes, which are discussed in this chapter.