Diabetes mellitus is a group of diseases associated with various metabolic disorders, the main feature of which is chronic hyperglycemia due to insufficient insulin action. Confirmation of chronic hyperglycemia is essential for the diagnosis of diabetes. HbA1c level is considered an indicator of the average blood glucose level over the last 1 to 2 months. According to the recent revision of diagnosis criteria for diabetes, in addition to the earlier listed plasma glucose values, HbA1c has been given a more prominent position as one of the criteria for diabetic diagnosis. Other biomarkers such as glycoalbumin and 1.5-Anhydro-D-glucitol are also useful for evaluating glycemic control. In this chapter, we provide an overview of each clinical biomarkers for diabetes.