C-peptide is known for several decades. It is released in equimolar amounts together with insulin from the pancreatic beta cells. Still there has been quite remarkable lack of interest in C-peptide. C-peptide is rarely used to classify type of diabetes although it seems self- evident that it is important to estimate the function of those cells which do not function good enough and therefore causes a syndrome which requires life-long treatment and leads to serious complications. Not until recent years C- peptide is accepted as a relevant outcome in trials aiming at preservation of beta cell function, although it is known for decades that some C-peptide is associated with less complications in type 1 diabetes (T1D). Preservation of beta cell function is important to make diabetes milder, and when beta cell function can be preserved before clinical manifestation of T1D, we are on our way to prevent that disease. Residual C-peptide/insulin secretion can be of value in classification of diabetes in different types. C- peptide may give valuable clinical information on why patients are more or less stable/labile in their blood glucose and more or less easy to treat. It explains why patients with T1D have different tendency to develop severe acute complications, both severe hypoglycaemia and diabetic keto-acidosis (DKA). Longstanding C-peptide may decrease risk of developing severe late complications. Finally, although still under debate, C-peptide seems to have several effects on different organs suggesting that it is an important hormone, interesting per se, and not only as a reflection of insulin secretion.