This chapter reviews the features of defective physiological changes of the spiral arteries in the placental bed in association with preeclampsia and fetal growth restriction and the methodology of placental bed vascular studies. The incidence of acute atherosis ranges from 41% to 48% in a series examining placental bed biopsies, placental basal plates, and amniochorial membranes. The basal plate of a delivered placenta is highly insufficient for the study of spiral arteries as it does not even represent the whole thickness of the decidua. The total number of spiral artery openings in the placental bed for a normal pregnancy was estimated at 120 and for severe preeclampsia 72. Indeed, examination of large hysterectomy specimens with placenta in situ has shown that in severe cases of preeclampsia a small, central part of the placenta may contain spiral arteries with fully developed physiological changes.