The role of the placenta in normal and pathological pregnancy is poorly understood, but important new insights into trophoblast biology have been obtained in recent years. The oxygen environment and retroviral envelope proteins are now known to play a role in morphological and functional differentiation of the human trophoblast. The qualitative and quantitative development of trophoblast hormonal functions during pregnancy, from a paracrine role during the first trimester to an endocrine role in placentation and uterine quiescence, is becoming increasingly clear. These findings are offering new perspectives in the diagnosis and treatment of disorders of placental origin.