Gestational diabetes mellitus (GDM) is defined as glucose intolerance with onset or first recognition during pregnancy. Data from Western countries suggest that the prevalence of GDM is increasing, being almost 10% of pregnancies and probably reflecting the global obesity epidemic. The majority of women with GDM seem to have β-cell dysfunction that appears on a background of chronic insulin resistance already present before pregnancy. In less than 10% of GDM patients, defects of β-cell function can be due to autoimmune destruction of pancreatic β-cells, as in type 1 diabetes, or caused by monogenic mutations, as in several MODY subtypes. Diagnostic criteria for GDM vary worldwide and there are no clear-cut plasma glucose cut-off values for identifying women at a higher risk of developing macrosomia or other fetal complications. Because the oral glucose tolerance test (OGTT) is restricted to high risk individuals, 40% of GDM cases are left undiagnosed. Therefore, in high risk populations almost universal screening is recommended; only women considered to have very low risk do not need screening. Diet and exercise are the key elements in the treatment of GDM. If necessary, either insulin, certain oral hypoglycemic agents or combinations can be used to achieve normoglycemia. After delivery, women with GDM and their offspring have an increased risk for developing the metabolic syndrome and type 2 diabetes. Thus, pregnancy may act as a “stress test”, revealing a woman’s predisposition to T2D and providing opportunities for focused prevention of important chronic diseases.