The population of the world has more than tripled during the twentieth century. The apparatus of technical demography has been fruitfully employed to study the causes and implications of this growth. However, at century’s end, fertility levels in Europe, North America, and East Asia have declined to levels below those necessary for long-term population replacement (United Nations, 1997). The tools of technical demography are also useful for studying growth prospects in low fertility populations, although they have been less frequently used for this purpose. This paper discusses two features of population dynamics in low fertility populations. The first involves a decomposition of the intrinsic rate of natural growth into a fertility and a mortality component. The lesson is that there are sharply “increasing returns” to fertility reductions in terms of their impact on long-term growth rates. A particular absolute decline in the number of children per woman will have larger and larger impacts on the rate of population growth as fertility levels fall. The second lesson is that, with reference to the situation that is or will soon be facing most of the industrialized world, there is a momentum to population decline that is exactly analogous to the better-publicized momentum of population growth. The first lesson merely involves presenting and interpreting some wellestablished formulas. The second takes advantage of several new expressions. Although migration is an additional factor of population growth in lowfertility countries, we choose to focus on fertility and assume that populations are closed to migration.