Viviparity combined with sustained invasive placentation is a unique reproductive innovation of placental mammals (Eutheria). This fact is remarkable as viviparity, of various kinds, is a quite frequent reproductive feature of animals in general. In this book, the authors argue that sustained invasive placentation faced a number of cell biological obstacles, known as biological paradoxa. In the evolution of eutherian mammals, each paradoxon was overcome by a specific evolutionary innovation that overcame the cellular biological constraint on the evolution of mammal-like viviparity. Each of these innovations also identifies specific mechanisms that entail clinical vulnerabilities of human pregnancy. The evolution of mammalian pregnancy is thus a paradigm of a significant evolutionary novelty, and this book exemplifies the conceptual tools necessary to explain evolutionary novelties in general, as well as the importance of an evolutionary and cell biological perspective on human reproductive health.