Ideas concerning the placenta, its development, structure and function can be traced to antiquity. An issue of importance that was only resolved in the late-eighteenth to early-nineteenth century is that of the separate nature of the maternal and fetal placental circulations. Other vital issues include the placentas of twins, the structure of placental villi and their covering, the classification of placental types, the existence of the intervillous space, and the relation of maternal to fetal blood flows in the placenta. Contemporary challenges in placental biology include understanding the basic mechanisms of numerous aspects of metabolism, endocrinology, immunology, epigenetics, and the role of specific placental proteins in embryonic/fetal development.