Preeclampsia, particularly the severe cases that occur early in pregnancy, is associated with defects in the (placental) cytotrophoblast differentiation pathway that leads to uterine invasion. At a morphologic level, interstitial invasion often is shallow. Perhaps more significantly, endovascular invasion, particularly the arterial component, is rudimentary. The latter defect is thought to lead to hypoperfusion of the placenta. At a molecular level, these defects are associated with particular deficits in the differentiation process whereby cytotrophoblasts--epithelial cells of ectodermal origin--assume vascular-like properties. Until recently, the question was how the latter defects could lead to the maternal symptoms of this condition. Now a possible link in the form of preeclampsia-associated changes in placental production of vasculogenic/angiogenic substances and their inhibitors has been discovered. It is likely that this new paradigm will improve both diagnosis and treatment of this life-threatening pregnancy complication.