This chapter presents a partial history of ideas about the maternal-fetal boundary of the gravid human uterus. It discusses the modern theoretical understanding of parent-offspring relations and how this predicts a complex interplay of cooperation and conflict. The chapter also considers how this interplay is expressed in relations between the endometrium and early human embryos. Prior to the late 1890s, the human embryo was not generally perceived as invading maternal tissues. Decidual reactions occur only in mammals with invasive placentation. Genetic variants that enhance fetal fitness and maternal residual fitness will be favored by natural selection regardless of whether they are expressed in the mother or the fetus. The evolutionary relations between a mother and a fetus are fundamentally similar to the relations between the same two genetic individuals after the birth of the fetus.