Gestational diabetes (GDM) is defined as any degree of glucose intolerance with onset or first recognition during pregnancy and is associated with increased feto-maternal morbidity as well as long-term complications in mothers and offspring. GDM is diagnosed by an oral glucose tolerance test (OGTT) or overt hyperglycemia. In case of a high risk for GDM or type 2 diabetes measurement of fasting glucose and HbA1c and/or performance of the OGTT (120 min; 75 g glucose) is recommended already in the first trimester and if normal the OGTT should be repeated in the second/third trimester. In case of clinical symptoms of diabetes (glucosuria, macrosomia) the test has to be performed immediately. All other women should undergo a diagnostic test between 24 and 28 gestational weeks. Based on the results of the hyperglycemia and adverse pregnancy outcome (HAPO) study an international consensus suggests new criteria for the diagnosis of GDM with the following plasma glucose values: fasting 92 mg/dl, 1 h 180 mg/dl and 2 h 153 mg/dl. In case of one pathological value a strict metabolic control is recommended. The women should receive nutritional counseling and be instructed in blood glucose self-monitoring. If blood glucose levels cannot be maintained in the normal range insulin therapy should be initiated. Maternal and fetal monitoring is required in order to minimize maternal and perinatal morbidity and mortality. After delivery all women with GDM have to be reevaluated as to their glucose tolerance by a 75 g OGTT (WHO criteria) 8 weeks postpartum and informed about their increased risk of development of diabetes and potential prevention strategies at follow-up.