At the time of the discovery of the physiological changes of spiral arteries in the pregnant uterus, Brosens and colleagues suggested that these changes result from the destructive action of the invading trophoblasts on the vascular smooth muscle and the elastic membrane. This chapter reiterates the main findings regarding the successive spiral artery remodeling steps. It seems appropriate to relate the time-course of the vascular remodeling process to the new insights in uteroplacental flow changes during this pregnancy period. In preeclampsia, trophoblast-associated remodeling is restricted to decidual spiral arteries throughout the placental bed. Spiral artery conversion is obviously important for safeguarding an adequate maternal blood supply to the placenta. Deep trophoblast invasion and spiral artery remodeling of the inner 'junctional zone' myometrium is a feature of normal human pregnancy, while in preeclampsia and maybe in other pregnancy complications this process may be seriously impaired.