Gestational diabetes mellitus (GDM), or diabetes first recognized during pregnancy, is associated with increased risk of adverse perinatal outcomes. After GDM delivery, both mothers and offspring are at risk for long-term chronic disease. Clinical recognition and treatment of GDM may reduce peripartum risk, but there is a lack of consensus on how to define, screen and treat GDM. There is also a lack of information on how treatment affects long-term outcomes as well as precise information on the prevalence of long-term outcomes. Consensus regarding the definition of GDM may be achieved in the next several years with completion of the HAPO trial, but further study on the long-term effects of oral medications other than insulin is needed, and greater attention needs to be devoted to improving postpartum screening and to the follow-up and intervention for women and offspring to improve long-term outcomes.