At present the diagnosis of gestational diabetes mellitus (GDM) is based on different criteria in locations around the world, and none are based on pregnancy outcome. Various glucose loads (50, 75 or 100g) are recommended, as opposed to the standard 75g load used worldwide in nonpregnant persons. It is controversial whether maternal hyperglycemia less severe than that in diabetes is associated with increased risks of adverse pregnancy outcomes. The HAPO Study was designed to address this question. A total of 25,505 pregnant women at 15 centers in 9 countries underwent 75g oral glucose tolerance testing at 24 to 32 weeks gestation. Data remained blinded if the fasting plasma glucose was < 105 mg/dl and the 2 hr plasma glucose was < 200 mg/dl. Among 23,316 women who remained blinded, each of the three OGTT values was significantly and continuously related to various adverse outcomes, with no inflection point. An international group of experts has developed recommendations for diagnostic criteria for GDM that are based on these results. This chapter provides an overview of the HAPO Study and the new recommendations.