Much of the world is aging rapidly. Both the number and proportion of people aged 65 years and older are increasing, although at different rates in different parts of the world. The number of older adults has risen more than threefold since 1950, from approximately 130 million to 419 million in 2000, with the elderly share of the population increasing from 4 percent to 7 percent during that period. In the United States, those aged 65 and older currently make up about 13 percent of the population. The US Census Bureau (2004) projects that in 25 years this proportion will exceed 20 percent. Over the next 50 years the United States will undergo a profound transformation, becoming a mature nation in which one citizen in five is 65 or older. (Now, one person in eight is that old.) The dramatic increases to come in the older population will exert powerful pressures on health care delivery systems, on programs such as Social Security, Medicare, Medicaid, and Supplemental Security Income that provide financial support, and on social institutions such as the family that provide instrumental, financial, and emotional support for the elderly.