Diabetes Mellitus is a chronic illness characterized by hyperglycemia that results from defects in insulin secretion, insulin action, or both. The prevalence of diabetes mellitus has increased in the United States. Decreased physical activity, increasing obesity, and changes in food consumption have been implicated in the disease process. Hyperglycemia is the hallmark for the development of chronic complications in patients with the disease. Long-term epidemiological studies have demonstrated that control of glucose to target levels reduces both primary and secondary complications of diabetes mellitus. This article briefly reviews the definition, classification, diagnostic criteria, management, and acute and chronic complications of diabetes mellitus.