This chapter provides difference between developed, less developed, and least developed nations, and give examples of each type of nation. It contains three measures of population aging used by demographers. The chapter talks about three demographic forces that lead to population aging in the United States. It describes the reasons for migration among the older population within the United States. The chapter discusses the use of dependency ratios as a way to measure of the cost of an aging society. The Chinese government has made some adjustments to its one-child policy to slow the speed of population aging. Demographic dividend the economic benefit a society gains from having a low birth rate, a proportionally large working population, and a small older population. A population ages when any of these measures increase. Populations with large numbers of older people or with high proportions of older people are said to be old or aging societies.