The placenta is a vital, temporary organ in eutherian mammals that facilitates the exchange of gases, nutrients, and waste products between the mother and developing fetus. This chapter explores the detailed structure and classification of the placenta, highlighting the diversity of placental types among mammalian species. Two primary criteria form the basis for classification: the gross morphological distribution of fetal-maternal contact sites and the number of tissue layers separating maternal and fetal blood. Placental types are described based on shape as well as histological intimacy. Comparative examples across species provide insight into evolutionary adaptations.