Altered fetal growth is associated with specific changes in placental amino acid transporters, which may cause changes in fetal growth rate and mediate programming of the fetus. This chapter reviews evidence suggesting that regulation of placental amino acid transporters constitutes a critical link between maternal nutrient availability and fetal growth and programming. Only system A and system L amino acid transporters have been studied and found to be present in multiple isoforms, highly regulated and altered in abnormal fetal growth. Placental functional capacity is likely to be subjected to regulation by fetal, placental and maternal factors. Maternal protein restriction in the rat is a well established model for the study of developmental programming. In obese women, pregnancy complications are more common than in lean women. There are a large number of signals impinging on the regulatory pathways that determine the expression and activity of the placental amino acid transporters.