The implantation process requires a functionally normal conceptus and a receptive endometrium, but also a communication link between them. This paracrine dialogue involves not only gonadal steroids but also a variety of other biologic molecules secreted by the conceptus and the reproductive tissues themselves in a communicative, interconnected network. The factors facilitating this dialogue includes chemokines, cytokines, adhesion molecules, growth factors. Recently, the soluble form of HLA-G and the vitamin D system have also been proposed as components of this cross-talk. Inherent among the factors involved is the fact that their effects are redundant and pleiotropic. Normal implantation and placentation are critical for a successful pregnancy. Therefore, a better understanding of the molecular mechanisms responsible for these processes will lead to the development of new regulating agents with novel diagnostic, biological and therapeutic potential for both facilitating and hindering a normal reproductive function.