The late 20th century brought the dawn of global population aging, the culmination of decades-long shifts to lower fertility and longer life expectancy. These novel age distributions—larger proportions of older persons relative to working-age or youth—bring with them a plethora of questions about the political, economic, and social causes and consequences of such aging. There are multiple theoretical perspectives and ways to measure population aging, and decisions about approaches, definitions, and measurements can make a dramatic difference on the results of studies of its impact. Some scholars approach the study of aging through a generational lens, others through chronological age, dependency ratios, or other measures of age structure. Studies of the implications of population aging fall into three major categories: political, economic, and social. Political demography studies often focus on the political power of various age groups and attempt to assess the degree to which intergenerational conflict is emerging as the sizes of age groups change and their demands on services like entitlements shift alongside. Political demography studies also look at voter behavior and preferences to assess possibilities for reform of age-related policies, like retirement, healthcare, and education. A separate branch of political demography examines the military implications of population aging, particularly its effect on the willingness and ability of a state to use force. Of the few studies that show a link between aging and war, empirical results are inconclusive, meaning that it is just as likely a state with a high median age will be belligerent as not. Studies on the economic implications of population aging look at the changing nature of the labor market itself and on the possibility of macroeconomic growth in the face of demographic change. Finally, research on the social impact of population aging is mostly concerned with individual- and family-level well-being, as the care demands of an aging population create pressures on individuals, families, and social safety nets. There are many controversies and debates over the impact of aging, including debates over the relative weight of demographic factors and whether population aging is a trend warranting celebration or alarm. In all, there are far more questions about the implications of aging than there are answers, and the projected development of this trend means that more questions constantly arise. Lingering questions surround historically rapid demographic aging, new sets of aging states at different speeds, shrinking populations, the intersection between migration and aging, and the intersection between aging and climate change. The field is ripe for more comparative aging work in general.