The main purpose of treating diabetes is to prevent chronic complications. Strict glycemic control is known to suppress the occurrence and progression of these complications. The test for plasma glucose is essential to identify diabetic patients, as mild hyperglycemia without symptoms can be a risk factor for complications. The new classification and diagnostic criteria for diabetes were proposed by the American Diabetes Association (ADA), WHO and Japan Diabetes Society (JDS) between 1997 and 1999. Diabetes is classified into four etiological categories; type 1, type 2, diabetes due to other specific mechanisms or conditions, and gestational diabetes. Another classification system according to the degree of metabolic abnormality has also been adopted. For diagnosis of diabetes, the JDS Committee classified the glycemic state into three categories based on fasting plasma glucose (FPG) and 2-h plasma glucose in the 75 g oral glucose tolerance test (2hPG); normal type (FPG <110 and 2hPG <140 mg/dl), diabetic type (FPG <126 and/or 2hPG <200 mg/dl), and borderline type (neither normal nor diabetic type). The borderline type corresponds to the sum of impaired fasting glycemia (IFG) and impaired glucose tolerance (IGT) based on ADA and WHO. Using the JDS criteria, diabetes is diagnosed when hyperglycemia of ‘diabetic type’ is confirmed on two or more occasions. ADA recommends the use of FPG alone for the diagnosis of diabetes, but findings from both Japan and Europe indicate that many diabetic subjects would be classified as non-diabetic solely on the FPG test. JDS recommends the use of the glucose tolerance test when the elevation of FPG is mild. Keeping glycemia near-normal by periodic monitoring of glycemic parameters and by appropriate treatment would prevent or reduce the diabetic complications in patients to a minimum.