Placental vasculature, in particular the relationship between maternal and fetal blood circulations, has been a contentious issue for a long time. In his magnificent Anatomy of the human gravid uterus William Hunter included the first drawing of spiral arteries (convoluted arteries), in what must have been the very first illustration of a human placental bed. The French anatomist Mathias Duval was probably the first to recognize the invasion of trophoblast into endometrial arteries, in this case in the rat. It it became clear that deep trophoblast invasion and associated spiral artery remodeling are essential for a healthy pregnancy. The actual depth of invasion was underrated for a long time, partly because of the increasing popularity of the decidual barrier concept. Early observations of trophoblast invasion into the spiral arteries set the stage for understanding the maternal blood supply to the placenta via the spiral arteries of the placental bed.