Insulin therapy is a vital hormone replacement therapy in type 1 diabetes mellitus. In type 2 diabetes, insulin is indicated if glycaemic goals are not reached by oral anti diabetics, as well as for metabolic detoriation, co-morbidities, surgery, pregnancy or contradictions against oral anti diabetics. Insulin preparations are characterized by the onset of the insulin action, the peak profile and duration of action. Available are short acting, long-acting and premixed preparations of human insulin, and insulin analogues. The gold standard of insulin therapy in type 1 diabetes is functional insulin therapy with a basal-bolus insulin regimen and control and adaption of the therapy by the patient. Various insulin regimens are available for treating patients with type 2 diabetes, including basal insulin supported oral therapy, supplementary mealtime injection of short acting insulin or insulin analogues, conventional insulin therapy or a basal bolus procedure. The various insulin preparations and regimens make it possible to adapt the therapy according to the patient's individual need.