Diabetes mellitus is characterized by impaired insulin secretion and varying degrees of insulin resistance, causing hyperglycemia. Early symptoms are associated with hyperglycemia and include polydipsia, polyphagia, polyuria, and visual field constriction. Late complications include angiopathy, peripheral neuropathy, nephropathy, and predisposition to infectious diseases. Diagnosis is based on the measurement of plasma glucose levels. Treatment includes diet, exercise, and glucose-lowering agents, including insulin, oral antidiabetic drugs, and non-insulin injectables. Complications can be delayed or prevented by proper glycemic control; Cardiac disease remains the leading cause of death in diabetes mellitus.