Knowledge of normal insulin needs and action is of great clinical importance in the case of insulin replacement in Type 1 diabetes, but also for appropriate timing of food intake and initiation of insulin treatment in Type 2 diabetic patients. Thus, hyperglycaemia in diabetes mellitus (DM) may be associated by primary insulin deficiency (DM Type 1) or by impaired insulin action (DM Type 2, early phase) or by both (unsubstituted insulin deficiency in DM Type 1; or DM Type 2 in its late phase). This review investigates to the pattern of insulin secretion in diabetic states and compares the observed phenomena with normal insulin release in healthy man. In particular, it is of note that B-cell function and temporal organization of insulin release is profoundly different and not normal in Type 2 diabetes though overall absolute insulin release (U/24h) may well be within or above the normal range. Such data are to caution the clinician against premature stimulation of endogenous insulin release by sulfonylureas or against too early initiation of insulin treatment in Type 2 diabetes.