As currently defined (5), gestational diabetes is associated with important perinatal and long-term health risks. Many of the risks increase, in relation to the severity of maternal hyperglycemia. For perinatal risks to infants, the relationship seems to be continuous. Maternal fasting glucose levels can be used to identify subsets of patients with very low and very high risks. The majority of pregnancies lie between these two extremes; and nonglucose measures, such as fetal ultrasound, can be used to enhance risk assessment, thereby minimizing over- and undertreatment of patients. The major long-term maternal risk is development of type 1 or type 2 diabetes, predominantly the latter. The risk increases continuously, in relation to maternal glycemia during and, especially, after pregnancy. Patients seem to have a B-cell defect that is characterized by maladaptation to insulin resistance. The B-cell defect is predictive of future diabetes, supporting the testing and clinical application of interventions that minimize insulin resistance to delay or prevent diabetes. Women with impaired glucose tolerance in the first few months postpartum are at particularly high risk for diabetes and should receive the most intensive education, intervention, and follow-up. Offspring of women with GDM are at increased risk for obesity and have an unexpectedly high prevalence of elevated glucose levels during childhood and adolescence. Both genetic and intrauterine environmental influences are likely to contribute to these abnormalities. Optimal strategies to detect and prevent the long-term risks to offspring remain to be established.