This chapter tries to identify some trends of placental evolution in primates by comparing placental organization between those with haemochorial placentas, with special attention to the great apes. Due to the possible relevance to placental pathologies and pregnancy complications, it particularly focuses on trophoblast invasion and uterine spiral artery remodelling. In Old World monkeys, a cytotrophoblastic shell is formed but remains intact throughout pregnancy, showing a clear demarcation from the underlying decidua where there is little interstitial invasion by extravillous trophoblast. Endovascular invasion starts much earlier in rhesus monkeys than in the human. Decidualization in the human is more extensive than in rhesus monkeys and baboons, and has been related to the deeper trophoblast invasion in our own species. A few cases of pre-eclampsia have been described in chimpanzees and gorillas, but no information is available about a possibly impaired invasion depth or defects in spiral artery remodelling.