The physiology of parturition in mammals is highly diverse and almost species specific. A range of examples is presented from the marsupial Tammar wallaby (Macropus eugenii) to the primate Homo sapiens. In some species, such as the sheep, parturition is initiated by the fetus while in others, for example the goat, parturition is initiated by the mother. In the human and other great apes parturition appears to be regulated by placental production of steroids from both fetal (estriol) and maternal (estradiol) origin. The role of progesterone related steroids and estrogens varies dramatically amongst species. In a number of mammals progesterone withdrawal initiates labor while in others a rise in estrogens is the precipitant. However, in other species, such as the Tammar, neither estrogens or progestagens appear to have any role. In all species examined to date prostaglandins play a role in the process of parturition although the regulation of increases in prostaglandins varies with differing prostaglandin synthases involved in different species and differences in the tissues of origin and sites of action. The enormous variation in the physiology of parturition is related to the high rates of perinatal death of both mother and offspring and the consequent evolutionary pressures that are exerted on this process.