This chapter describes the biological processes and some of the molecular mediators required during early implantation events and illustrates the clinical consequences when these processes are perturbed. Implantation occurs at approximately 6-7 days after fertilization. Initial adhesion of the blastocyst to the uterine wall, termed apposition, is unstable. The temporal and spatial expression of several growth factors, cytokines and adhesion molecules within the uterus and pre-implantation blastocyst suggests that they may play important roles. Reproductive pathologies resulting from implantation defects span a spectrum of clinical presentations ranging from infertility to recurrent pregnancy loss to pre-eclampsia. Infertility may result from failure of fertilization or from loss of the fertilized blastocyst prior to implantation. Pre-eclampsia, a clinical syndrome characterized by hypertension and proteinuria that develops after 20 weeks' gestation, is the leading cause of maternal mortality in the industrialized world and increases perinatal mortality five-fold.