Population momentum refers to population growth at the national level which would occur even if levels of childbearing immediately declined to replacement level. For countries with above-replacement fertility (greater than 2.1 children per woman), population momentum represents natural increase to the population. For below-replacement countries, momentum corresponds to a population decline. Momentum occurs because older cohorts differ in absolute size from those cohorts currently bearing children, which impacts the immediate birth and death rates in the population which determine the intrinsic rate of growth. Formal demographers refer to population momentum as the size of the resulting stationary-equivalent population relative to the current size of the population. Population momentum has implications for population policy for a number of reasons. First with respect to high-fertility countries in the developing world, population momentum instructs us that these countries will continue to grow despite large and rapid declines in fertility. Second with respect to lowest-low fertility countries in Europe, momentum implies that these countries may experience population decline even if they bring their fertility levels up to replacement. In fact, population momentum can become negative if fertility rates are under replacement for long enough; in coming decades, for example, some Eastern European countries would show population shrinkage even if their birth rates recovered to replacement level. Finally, population momentum shows us that replacement level fertility is a long term concept rather than an indication of current population growth rates. Depending on the extant age structure, a fertility rate of 2 children per woman may correspond to either short-term growth or decline.