The rapid population aging that faces much of the world represents a historically unprecedented demographic shift with profound social, political, and economic implications. With funding from the Stanford Center on Longevity (SCL), PRB has collaborated with the Global Aging Program at SCL on the development of the SCL/PRB Index of Well-Being in Older Populations, a composite index to measure the well-being of older people across countries. The SCL/PRB Index provides a much-needed summary measure of the complex and multidimensional aspects of how well older adults are doing. The lack of a standard summary index of well-being for older populations has hampered ongoing assessments of policies and programs aimed at helping individuals as they age. The SCL/PRB Index provides comparable measurement of well-being for older age groups for the United States and 11 countries in Europe (Austria, Belgium, Denmark, France, Germany, Greece, Italy, Netherlands, Spain, Sweden, and Switzerland). The well-being in older populations is based on 12 indicators spanning across four domains: material, physical, social, and emotional well-being. The SCL/PRB Index allows for cross-national comparison of the overall well-being of older population groups—at ages 50 to 64, 65 to 74, and 75 and older—in these 12 countries. This will enhance the ability of decisionmakers to assess the effects of different social, political, and policy contexts on the well-being of older populations.