This chapter provides a general overview of the two major components of pregnancy-associated spiral artery remodeling, the maternal (decidual) and the fetal (trophoblastic). During placentation the mother comes in close contact with semi-allogeneic fetal trophoblastic cells which play a key role in maternal-fetal physiological exchange. Focusing on spiral artery invasion, there is little direct evidence for the occurrence of intrinsic trophoblastic defects in early pregnancy. Impaired trophoblast invasion in spiral arteries may not only be due to an intrinsic defect in invasive properties, but may also be induced by maternal cells. For a long time trophoblast invasion was thought to be controlled by a restrictive action of the decidua. A stimulatory role of decidua for trophoblast invasion may also apply to the endovascular invasion of the spiral arteries. Decidua-associated vascular remodeling not only occurs in the decidua but also in the junctional zone myometrial compartment.