Gestational diabetes mellitus (GDM) is defined as glucose intolerance with onset or first recognition during pregnancy. As such, GDM is the product of routine glucose tolerance screening that is currently carried out in otherwise healthy individuals. Like other forms of hyperglycemia, GDM is characterized by pancreatic β-cell function that is insufficient to meet the body's insulin needs. Available evidence suggests that β-cell defects in GDM result from the same spectrum of causes that underlie hyperglycemia in general, including autoimmune disease, monogenic causes, and insulin resistance. Thus, GDM often represents diabetes in evolution and, as such, holds great potential as a condition in which to study the pathogenesis of diabetes and to develop and test strategies for diabetes prevention. The clinical detection of GDM is accomplished in different ways in different countries. In general, the approaches apply one or more of the following procedures: 1 ) clinical risk assessment, 2 ) glucose tolerance screening, and 3 ) formal glucose tolerance testing. The procedures are applied to pregnant women not already known to have diabetes. Controversies regarding the optimal methods for detecting GDM are beyond the scope of this article. The relevant point is that the screening for GDM is the only standard medical practice that applies screening for glucose intolerance to otherwise healthy individuals. Regardless of the glucose thresholds that are used to diagnose GDM, the patients are relatively young individuals whose glucose levels are in the upper end of the population distribution during pregnancy. A small minority of those women have glucose levels that would be diagnostic of diabetes outside of pregnancy. The large majority have lower glucose levels when they are diagnosed with GDM, but they are at high risk for developing diabetes after pregnancy. Together, patients with GDM offer an important opportunity to study the evolution of diabetes and to develop, …