How implantation is regulated and brought about remains an enigma. The objective of the present review is to propose a hypothetical model based on our actual knowledge of the role of cytokines, adhesion molecules and invasive proteinases in the adhesion and invasion phases of embryonic implantation. For the purpose of clarity we will describe first what is known about the presence of these molecules in the endometrium and embryo in animal models and their relevance in embryonic implantation. Secondly, since human implantation is unique the presence of these factors in the human endometrium and embryo and their possible clinical relevance is considered separately. Finally, the link between cytokines, adhesion molecules and invasive proteinases as well as their endocrine/paracrine/autocrine regulation is put forward as a possible model that could lead us to a better understanding of the implantation process in humans.