Gestational diabetes mellitus (GDM) is defined as hyperglycemia first detected during the second or third trimester of pregnancy. It is diagnosed with an oral glucose tolerance test (OGTT), administered at 24–28 weeks’ gestation and universal screening is recommended for all pregnant women. Earlier testing in the first trimester is encouraged for women with risk factors for diabetes. Screening for GDM starts with a 1-hour, 50 g nonfasting screening test followed by a 3-hour, 100 g OGTT if the screening test is positive. Upon diagnosis, women should receive nutritional and exercise education and begin blood glucose monitoring. Women should target glucose levels below 95 mg/dL fasting and below 140 mg/dL at 1 hour or 120 mg/dL at 2 hours postprandially. Treatment of GDM reduces the risk of many of its complications. If glucose levels are not at target with lifestyle modifications, medical management should be initiated. Weight-based insulin is the recommend first-line therapy. However, metformin, an oral antidiabetic agent, can be considered in some cases. Gestational diabetes is an antecedent to type 2 diabetes and all women with GDM should be tested for underlying diabetes with a 2-hour 75 g OGTT at 4–12 weeks postpartum.