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C-peptide (connecting peptide) connects alpha and beta chains of proinsulin, which are formed in the endoplasmic reticulum following the removal of the signal peptide of pre-proinsulin. It is secreted from the beta cells of islets of Langerhans of the endocrine pancreas when proinsulin is cleaved into insulin and C-peptide. It plays an important role in the correct folding of insulin and the formation of disulfide bridges. C-peptide is removed in the Golgi apparatus from proinsulin resulting in the formation of the mature insulin molecule with both alpha and beta chains bound together by disulfide bonds.[1] Both insulin and C-peptide are stored in secretory vesicles and released in equimolar concentrations upon stimulation of beta cells by glucose and other secretagogues.[2] The most important indications for measurement of C-peptide levels include the differential diagnosis of fasting hypoglycemia with hyperinsulinism and as a measure of insulin secretory reserve. This brief review covers the relevant biochemistry, measurement, and clinical indications.