Examination of the placenta must be thorough, and frequently is accomplished best in the delivery room. Abnormalties of development and implantation include succenturiate lobe, placenta membranacea, placenta marginata, placenta circumvallata and marginal or membranous insertion of the umbilical cord. Gross examination of twin placentae may allow distinction between single ovum or double ovum twinning, but microscopic examination is more helpful. Degenerative changes normally occur in the pla-cena with maturity. Infarction of greater than 30% of the placenta is associated with increased perinatal loss. A placenta may be affected by a variety of maternal diseases, the most important of which is intra-uterine infection. Toxemia of pregnancy is associated with premature aging of the placenta, and maternal vascular changes. The placenta of erythroblastosis shows microscopic evidence of relative immaturity. Tumors are rare abnormalties of the placenta. Studies of placental physiology should be extended to include specific morphologic abnormalities.