The placenta serves as the fetus' organ of gas exchange throughout intra-uterine life. While the dependence of fetal well-being on an intact maternal-placental unit has been recognized for centuries, it is only in the last several decades that research with fetal animals has begun to unravel the mechanisms by which it regulates blood supply and oxygen, as well as its role in the maternal-to-fetal transfer of carbohydrates, proteins, fats, water, and inorganic salts. The anatomy and physiology of the placenta are presented here as they relate specifically to gas exchange. In addition, compensatory adaptations of the fetus and placenta to acute asphyxial events will be discussed.