One of the major characteristics of the heterodimeric amino acid transporters is their broad substrate selectivity. As a result of this property these transporters are of particular importance in that they likely are responsible for the majority of amino acid supply to the fetus. Fetal growth is intimately linked to placental transport of amino acids and both in vitro and in vivo studies have shown that the activity of placental heterodimeric amino acid transporters are decreased in IUGR [4,5]. Thus, detailed information on the molecular biology and function of placental heterodimeric amino acid transporters will not only increase the understanding of the regulation of fetal growth in normal pregnancies and in pregnancy complications, but may also provide a foundation for designing novel avenues of intervention in addressing altered fetal growth.