Since the early 1960's the world has witnessed the spectacular collapse of human fertility. As a result of this phenomenon several countries are already seeing their population numbers fall and more will follow in the coming decades. The causes of this fertility decline involve a complex interplay of socioeconomic, environmental, and biological factors that have converged to constrain fertility in posterity's wake. Since large numbers of offspring are no longer needed to compensate for high infant mortality in contemporary society, couples have opted to have small families in a quality-over-quantity investment in their progeny's future. Simultaneously, increases in female education, the enhanced participation of women in the paid workforce, and a resultant delay in childbearing has placed limits on achievable family size. Progressive urbanization, the improved availability of contraceptives and the socioeconomic pressures experienced by young adults in ageing societies, are also contributing to fertility's demise. These factors together with the individualism that pervades modern society and the increasing social acceptability of voluntary childlessness, have firmly established a low fertility ethos in most post-transition countries. Since none of these forces are about to relent, it looks as if extremely low fertility might be with us for some time to come. This may have long-term consequences. The lack of selection pressure on high fertility genotypes, the ability of ART to retain poor fertility genotypes within the population and sustained exposure to reproductive toxicants in modern industrialized environments, may all contrive to leave a permanent mark on the fecundity of our species.