It is very important test.The oral glucose tolerance test (OGTT) has been the mainstay for diagnosing diabetes for decades. Recently, the American Diabetes Association (ADA) suggested abandoning the OGTT, while resorting to a simpler screening test, exclusively based on baseline fasting blood glucose concentration. This review article rewinds the history of OGTT and its recent advancements, and compares its power in detecting early diabetes with that of fasting blood glucose alone. The key point is that there are more diabetics originating from a population with normal fasting blood glucose than from subjects with impaired fasting glucose, those who can be detected by the new ADA recommendations. Conversely, the OGTT detects more efficiently early diabetes as well as subjects with IGT, as the glycemia at the second hour seems crucial as a diagnostic tool. We discuss the different significance of fasting versus second hour glycemia during OGTT, according to different mechanisms of glucose homeostasis. Finally, we provide recent evidence on very simple additional information that can be obtained from the OGTT, which renders this test even more useful, discussing pathophysiologic significance.