Implantation in the human is unique. This uniqueness is characterized on the maternal side by a spontaneous and massive decidualization of the endometrium and on the embryonic side by an almost unlimited invasive potential. Human embryos express an intrinsic invasive potential, which allows them to implant almost anywhere except in the endometrium because it protects itself from implantation. Human implantation is thus only possible during a limited period of time known as the implantation window. This mini review stresses the importance of studying trophoblast invasion into the endometrium as a model for human implantation. Cytotrophoblastic cells (CTB) can easily be isolated from first-trimester legal abortions and retain their invasive behavior when cultured in vitro. This model shows that matrix metalloproteinases (MMPs) are produced by CTB and are instrumental to their invasive behavior. Embryo implantation and tumor invasion use these same biochemical mediators for invasion. However, in contrast to tumor invasion, trophoblast invasion is limited both in time and space: it occurs during the first trimester of pregnancy and invasion does not go beyond the proximal third of the myometrium. Factors regulating MMP expression are of maternal and fetal origin.