We are in the midst of an unprecedented transition in global demography. The world's population is aging rapidly, and older adults compose a larger proportion of the world's population than ever before-a share that will only increase over the next century. By 2050, the percentage of the United States' population that is aged sixty years and older will grow from the current ½gure of about 20 percent to 27 percent. The global number of centenarians worldwide-those aged one hundred years and older-is expected to more than double by 2030, with projections of nearly 3.4 million by 2050. 1 Three major factors are driving this transition: decreasing fertility, increasing longevity, and the aging of large population cohorts.Falling fertility rates are the main determinant of population aging. Low fertility rates lead to smaller youth cohorts, which create an imbalance in the age structure: older age groups become larger than their younger counterparts. Thanks to accessible and effective birth control, increased child survival, and cultural changes, birth rates have dropped dramatAbstract: The rapid aging of populations around the world presents an unprecedented set of challenges: shifting disease burden, increased expenditure on health and long-term care, labor-force shortages, dissaving, and potential problems with old-age income security. We view longer life spans, particularly longer healthy life spans, as an enormous gain for human welfare. The challenges come from the fact that our current institutional and social arrangements are unsuited for aging populations and shifting demographics; our proposed solution is therefore to change our institutions and social arrangements. The ½rst section of this essay provides a statistical overview of global population aging and its contributing factors. The second section outlines some of the major challenges associated with widespread population aging. Finally, the third section of the essay describes various responses to these challenges, both current and prospective, facing individuals, businesses, institutions, and governments.