Gestational diabetes mellitus (GDM) is a type of diabetes that presents during pregnancy and usually disappears shortly after a woman gives birth. Better recognition of the risk factors of GDM, combined with more universal screening for the disease in many countries, has led to the increased detection of GDM along with other forms of pregestational diabetes. There is growing evidence that GDM significantly increases the risk of a number of short- and long-term adverse consequences for the fetus and mother, the most significant of which is a predisposition to the development of metabolic syndrome and Type 2 diabetes. Maternal and childhood obesity as well as cardiovascular disease are also potential long-term consequences of GDM. On the other hand, there is a growing body of evidence suggesting that the risk of many of these consequences can be significantly reduced or eliminated by aggressive treatment of GDM. There remains, however, a great deal of controversy over when to begin screening for hyperglycemia in pregnancy and at what level of hyperglycemia should aggressive intervention be initiated.