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Population aging refers to an entire age structure becoming older. The age structure of a population is the result of three basic processes: fertility, mortality, and migration. Age structures reflect both past effects and current patterns of these processes. At the town, city, or regional level, migration becomes an important factor in raising the age level of the population, but in general the age structure of countries is more dependent on fertility and mortality. High fertility and high mortality ultimately produce a young population, while low fertility and low mortality in the long run produce an older population. A shift from high fertility and mortality to low fertility and mortality has accompanied industrialization in the developed world. Typically, mortality among the youngest age group declines first, resulting in a younger population. The population aging process begins when fertility declines. As mortality declines among the older age groups, the population ages further. It takes some time for the population to reach a stable age structure reflecting low fertility and mortality. Population aging around the world is inevitable for the 21st century. The demographic transition will require adjustment from social institutions. Four countries that exemplified the four stages of aging in 1985 are discussed (Nigeria, Brazil, Korea, and West Germany). Fifteen charts, graphs, and tables are provided to illustrate various aspects of this population aging process. A 9-item bibliography is included. (JB)