'Incretin' hormones has lately attracted considerable attention as new strategy improving insulin secretion of type 2 diabetes mellitus. Spending many years for study of incretin hormones, GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1), we can use incretin mimetics as new drugs for type 2 diabetes clinically today in Japan. In this section, we describe the concept and the history of incretin hormones.