Women diagnosed with gestational diabetes mellitus (GDM) are at substantially increased risk of developing type 2 diabetes and obesity, currently at epidemic rates in the United States. GDM, therefore, identifies a population of women at high risk of developing type 2 diabetes and thus provides an excellent opportunity to intervene years before the development of this disorder. It is well recognized that acute as well as chronic physical activity reduce fasting plasma glucose as well as improve glucose tolerance in type 2 diabetes. Recent studies have suggested that women with higher levels of physical activity have reduced risk of GDM. Therefore, we will test the hypothesis that an exercise intervention is an effective tool for preventing GDM among women with a history of GDM.