The level of fertility in a population is the principal determinant of the shape of its age structure, which in turn is a critical factor in the terms of trade within a pay-as-you-go system of public pensions. Fertility has fallen below the replacement level of 2.08 children per woman in all developed countries. Partly because of its age structural consequences, a majority of governments in these countries now respond to United Nations questionnaires by saying that their fertility is “too low” (Kohler et al. 2006). This paper reviews the major factors that appear to be affecting fertility levels in the United States, with an eye towards making defensible statements about future directions of fertility. The subject covers a vast disciplinary range including demography, economics, sociology, public health, reproductive biology, evolutionary biology, political science, and psychology. These disciplines have not produced a widely-accepted framework for analyzing the determinants of fertility at the level of a population. At various times, low fertility has been blamed on affluence and on economic downturns; on hedonic individualism and on archaic family values; on women’s economic independence and on women’s adherence to traditional roles. In the absence of powerful and successful theory, we will pursue an eclectic, inductive approach, surveying the landscape of fertility variation in search of clues about its principal drivers.