Processes of conceptus implantation and placentation vary among mammalian species. However, differences in physiological and biochemical processes were once thought not to differ so much, particularly as to the kinds of genes expressed. In fact, recent progress has identified that in addition to the hormones, cytokines, proteases and cell adhesion molecules classically characterized, epithelial-mesenchymal transition (EMT), epigenetic regulation and the expression of endogenous retroviruses (ERV) are all required for the progression of conceptus implantation to placentation. Thus, continued research into EMT, epigenetic regulation and the expression of ERVs will aid in enhancing understanding of their impact on reproductive physiology in humans and domestic animals.