**Preface to "Embryo Implantation and Placental Development"**

Human reproduction has been defined by many as a highly inefficient biological process. This is a consequence of the numerous coordinated and finely regulated molecular pathways which drive the different stages of embryonic development, from fertilization to implantation and placentation. Synchronization between the acquisition of blastocyst competence and uterine receptivity appears to be a limiting step in implantation, and the timely release of growth factors and cytokines is a key basis for a successful pregnancy. Over the last few years, many of the players involved in the regulation of such an intricated process have been identified, and their altered expression in pathological conditions has been recognized in some cases. The role of novel factors is emerging, possibly opening new paths for the interpretation of what is still defined as idiopathic infertility.

> **Luisa Campagnolo** *Editor*
