Thyroid hormones are critical to growth and development of the human fetus. In particular, the fetal central nervous system is extremely sensitive to the actions of the active ligand, tri-iodothyronine (T3). The placenta is the organ during pregnancy that allows transport between the mother and her baby by close interaction of the maternal and fetal circulations. Endogenous fetal thyroid hormone production does not occur until the beginning of the second trimester. However, there appears to be transplacental transport of thyroid hormones to the fetus earlier in gestation and organs, such as the central nervous system, appear to be exquisitely sensitive to their actions. The content of this Thesis describes my work, published in peer reviewed papers over the last fifteen years. It outlines the molecular mechanisms controlling the delivery and actions of thyroid hormones to the fetus.