Most countries in the world experienced a major increase in life expectancy during the 20th century and a resulting aging of their populations. Further gains in life expectancy are uncertain, particularly in developed countries already characterized by a high longevity, and little is known concerning the health state of future generations of the elderly. But there is no doubt that further population aging will produce larger numbers of older persons both in developed and developing countries. As the prevalence of most chronic diseases is high in old age, population needs change rapidly in health care systems still organized essentially to provide acute care to children and young adults. Old age is heterogeneous, but a large proportion of older persons is affected by multiple chronic diseases, resulting in a wide range of needs. Health systems will have to adapt to this new situation while still providing appropriate responses to acute diseases affecting all ages. Although future needs are difficult to quantify, their nature is already apparent. Providing for these needs will require a major investment in manpower, a diversification of services delivered by health care systems, changes in the training of health professionals and extensive research to define effective treatments for elderly patients with multiple co-morbidities.