The degree of trophoblast invasion is very variable between species . However, the factors influencing it, and benefits deriving from it, are little understood. In the epitheliochorial placentation of pig, camel, and horse, trophoblast microvilli interdigitate with those of the maternal uterine epithelium, and there is no trophoblast invasion as such. One modification of this is synepitheliochorial placentation where some specialised trophoblast cells detach and migrate through to the maternal epithelium and fuse with it, as is seen in ovine and bovine placentae. Another is the trophoblast girdle cell invasion into the endometrium to form the cups in the early horse placenta. In endotheliochorial placentation there is some invasion into the maternal stroma, stopping short at maternal blood vessels, while the most invasive form is haemochorial, found in human, rodent, bat and other species, where the maternal endothelium is stripped away so that the trophoblast is in direct contact with maternal blood. There is at present some controversy about which form is the most primitive and what evolutionary pressures influenced the development of the different placental types [2,3]. In order to understand these systems and their relevance to human pregnancy, it is important to examine the modes of placentation of various other species. The contributors to this workshop covered many different facets of this subject as well as a wide range of animals, from the elephant to the mouse. © 2006 IFPA and Elsevier Ltd.