Rapid population aging is occurring in many parts of the developing world. Age structures are shifting from a relative concentration of younger to older individuals. Formal and informal health care needs across the developing world are changing concurrently. Therefore, population aging has enormous implications for health and social policy. This essay, which serves as an introduction to a special issue of Journal of Cross-Cultural Gerontology, highlights several critical research topics that require attention due to their implications for the health of individuals living in developing countries that are experiencing population aging. These include: population health levels, trends, and individual health transitions; influences of socioeconomic status on health and the consequences of rapidly changing socioeconomic structures for population health; and comparative studies on health and aging. Comparative research, in particular, has been underdeveloped and underutilized, but has great potential for providing insights into health determinants as well as the uniformity versus variation of the aging experience across societies. The remaining four papers that make up this special issue deal with these research topics and together highlight the complexity that exists in assessing individual and population health trends in developing countries that are undergoing population aging.