Preeclampsia (PE) is one of the most severe complications of pregnancy. PE is responsible for the highest rates of morbidity and mortality for both pregnant women and the neonate. In this review, we first address general aspects of PE and its diagnosis, along with some epidemiological aspects of this disease in the mexican population, in particular the experience from the Instituto Mexicano del Seguro Social. Even though over the last 20 years a great deal of evidence has accumulated regarding PE's pathophysiology, an exact mechanism to explain its etiology has not been established. This review aims to cover the status of two of the most important hypotheses in the etiology of PE: the immunological and the placental ischemia hypotheses. Recent data suggest that Natural Killer cells (NK) play a major role in the decidual spiral arteriole remodeling and in normal placental development. In genetic studies, KIR receptors present in NK cells have been involved in the susceptibility for the disease. In this review, we discuss data of our group regarding the presence of NK cells in the decidua, at the end of pregnancy and the genotypes of KIR receptors in normal and preeclamptic Mexican population. PE is characterized by abnormal placentation and hypoxia with an increase of anti-angiogenic factors; the Hypoxia-inducible factor 1-alfa (HIF1-alfa) is over expressed in PE. In this review, we also included some of our results concerning the polymorphisms and regulation of HIF in preeclamptic women.