This article reviews carbohydrate and fat metabolism in both healthy pregnant women and women with gestational diabetes. Emphasis is placed on more recent investigations that have utilized stable, nonradioactive isotopes with insulin clamps to study gestational fuel metabolism. In early pregnancy, glucose-stimulated insulin secretion is increased, insulin sensitivity is unchanged or enhanced, and glucose tolerance is normal or slightly improved. Late gestation is characterized by accelerated fetal growth, rising concentrations of several diabetogenic hormones, and increased insulin resistance. The increased resistance reduces maternal glucose utilization, sparing carbohydrates for the rapidly growing fetus. The inhibitory effect of insulin on the rate of lipolysis is also significantly reduced during the third trimester of pregnancy. An earlier than normal switch from carbohydrate to fat utilization serves to promote the use of lipids as a maternal energy source. Women with gestational diabetes have been reported to have either comparable or increased insulin resistance during late gestation with several studies also demonstrating reduced insulin secretory capacity.