The article reviews gestational diabetes mellitus, including etiology, diagnostic and screening criteria, risk factors, and care of the affected woman. Gestational diabetes mellitus affects approximately 7% of all pregnant women, resulting in more than 200,000 cases each year, and is defined as glucose intolerance that begins or is first recognized during pregnancy. Women are considered at high risk for gestational diabetes if they are markedly obese, have a personal history of gestational diabetes, have a strong family history of diabetes, or have glycosuria. Risk assessment is essential in determining whether a woman should be screened or tested for gestational diabetes. Women who have had gestational diabetes should have comprehensive preconception care prior to a subsequent pregnancy to ascertain appropriate weight, nutrition, exercise, and signs of gestational diabetes.