We have reviewed the paracrine communication between the blastocyst and the endometrium during the implantation process. We know that the arrival of the human blastocyst at the implantation site triggers a cascade of molecular events that overlap those events imposed by the hormonal regulation. Some known and unknown molecules take part in this paracrine dialogue, enzymes like (COX-2), and other factors such as HB-EGF, LIF, or the IL-1 system are part of the system used for this process. The blastocyst is able to modulate endometrial receptivity by up or downregulation of different molecules such as integrins and mucins (MUC-1). The blastocyst also plays an important role in the induction of apoptosis in EEC, as a mechanism to breach the epithelial barrier, and also in the invasion phase. More studies in this direction are needed for the whole understanding of this dialogue between the two players in the process known as embryonic implantation.