Type 1 diabetes mellitus is a heterogeneous metabolic disease characterized by destruction of the pancreatic beta cells resulting in an absolute deficiency of insulin secretion with subsequent hyperglycemia. This review details the definition and classification, epidemiology, pathophysiology, pathogenesis, prevention, diagnosis, and management of type 1 diabetes mellitus. Figures show the opposing actions of insulin and glucagon, particularly within the liver, on substrate flow and plasma levels; plasma glucose, insulin and C-peptide levels; the structure of human proinsulin; the cellular actions of insulin; measurement of insulin levels after the administration of glucose; the pathways that lead from insulin deficiency to the major clinical manifestations of type 1 diabetes mellitus; the pathogenesis of type 1 autoimmune diabetes mellitus; the relationship between hemoglobin A1C and calculated average glucose level; basal-bolus and insulin pump regimens; and management of diabetic ketoacidosis. Tables list the etiologic classification of diabetes mellitus, criteria for the diagnosis of diabetes, American Diabetes Association standards for glycemic control in diabetes mellitus, insulin preparations, potential advantages of continuous subcutaneous insulin infusion compared with multiple daily injections, cardiovascular risk factor screening and treatment, and typical admission laboratory findings and monitoring in diabetic ketoacidosis. This review contains 10 highly rendered figures, 7 tables, and 66 references.