It is only relatively recently that attention has been directed to studies of the uterine side of the placenta to look for possible defects that might explain otherwise inexplicable pregnancy complications. Preeclampsia and intrauterine fetal growth retardation are two such disorders, in which new information has come to light by the study of placental bed biopsies and occasional cesarean hysterectomy specimens. It will be less easy to apply these techniques to such problems as spontaneous abortion and antepartum hemorrhage, but reemphasizing what should be the self-evident importance of the establishment and development of the uteroplacental blood supply might help reorient thinking about these and other important complications of pregnancy. Fresh thoughts are required too about the etiology and natural history of ectopic pregnancy, not so much for its own sake but more because of what it tells us about nidation and placentation in general. It is now difficult to insist on stringent criteria for the endometrium in human gestation, with all that this implies for the woman under investigation for infertility, when in some circumstances these criteria are flouted in what should be an alien mucosa. Much more needs to be known about the promotion and control exercised over trophoblastic differentiation and migration and interaction with uterine tissues. The uterus certainly can no longer be considered an immunologically privileged site even were that privilege extended to the fallopian tube and, indeed, to the adnexa generally. The constraining influence of decidua, if indeed it has such a property, requires elucidation; it cannot be fortuitous that only in species with hemochorial placentation, characterized by migratory nonvillous trophoblast, is a true stromal decidua formed. This surely indicates that decidua has a major role to play in negotiating" the treaty of compromise" ultimately signed between fetal and maternal tissues and if such a treaty is not signed, or is broken, defective placentation and its consequences must follow.