The dynamics of population aging will broadly shape future social and economic conditions in the United States and in many other developed countries. Although the quantitative dimension of population aging is easily characterized, the associated forces producing changes in health and functioning among individuals are less well understood. For 14 years the Milbank Quarterly, under the editorship of David Willis, fostered research into the interaction of demographic, epidemiological, public health, and clinical factors in order better to understand population aging. The Quarterly spurred the collection of longitudinal data, based on representative samples of the U.S. elderly and oldest-old populations, which were crucial to efforts to answer critical questions about anticipated changes in health and functional status as the population ages. This research will give us a better grasp of the policy and service implications of an aging population.