Aging populations are generating both challenges and opportunities for societies around the globe. Increases in longevity and improvements in health raise many questions. What steps can be taken to optimize physical and cognitive health and productivity across the life span? How will older people finance their retirement and health care? What will be the macroeconomic implications of an aging population? How will communities be shaped by the shift in age structure? What global interconnections will affect how each society handles the aging of its population? To address these questions, the National Academies organized a symposium, summarized in the present volume, to determine how best to contribute to an evidence-based dialogue on population aging that will shape policies and programs. Presentations in the fields of biology, public health, medicine, informatics, macroeconomics, finance, urban planning, and engineering approached the challenges of aging from many different angles. The presenters reviewed the current state of knowledge in their respective fields, identifying areas of consensus and controversy and delineating the priority questions for further research and policy development.