**4. Conclusions**

Over the last decades the research aimed to reveal the biomolecular processes and pathways underlying animal and human implantation has greatly progressed for two major reasons. On one hand, the exciting advances in available technologies allowed to define in depth the factors and the pathways involved in proper implantation. On the other hand, the introduction of ARTs and their spectacular development in response to the increasing clinical demands from infertile patients allowed to better understand the determinants of a successful implantation and of several conditions of reproductive failure. Additionally, the large diffusion of ARTs provided new perspectives for studies on implantation, making available biological samples previously unavailable for research; follicular fluid, granulosa cells, oocytes, embryos, culture medium of embryos, and blastocysts are examples of this.

As a general concept, it has become clear that reproduction in humans can be considered a rather inefficient process and in several ways is different from reproduction in other species. An emerging concept is that the proper molecular crosstalk between endometrium and blastocyst is of paramount relevance to ensure a proper implantation. In this context, the studies on animal models, apart for the above differences, may greatly help to increase current knowledge. The specific roles of blastocyst and endometrium are being discovered, although much progress still has to be done in this field. The final objective of this field of research effort is twofold: (1) to improve the understanding of how reproduction and implantation evolved and differentiated among the species; (2) to offer more and more effective treatment options to patients with infertility, RIF and RPL.

**Author Contributions:** Conceptualization, L.C. and M.M.; Writing—Original draft preparation, M.M.; Writing—Review and editing, L.C., C.T. and R.R.; visualization, V.L. and F.L.C.; supervision, L.C.; project administration, L.C. and M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been supported by the Grant for Fertility Innovation 2017, funded by Merck KGaA.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
