The placenta plays a key role in pregnancy, mediating exchanges between mother and fetus and maternal tolerance of fetopaternal antigens. In some species, it also produces hormones that ensure the maintenance of gestation and fetal well-being. This unique organ also has considerable potential for use as a model for various aspects of biology. Indeed, the use of transgenic mouse models has greatly improved our understanding of the genetic control of placental development in this species and has opened up new fields of investigation in developmental biology. Analogous cell types have been identified among human and murine trophoblasts: proliferative trophoblastic cells, invasive trophoblastic cells and cells differentiating into syncytium, but human and mouse placentas differ in both morphogenesis and endocrine function. Herein, the similarities and differences between the human and mouse models are reviewed, with a view to encouraging caution in the extrapolation of results from one model to the other.