The median age of the world's population is increasing because of a decline in fertility and a 20-year increase in the average life span during the second half of the 20th century. These factors, combined with elevated fertility in many countries during the 2 decades after World War II (i.e., the "Baby Boom"), will result in increased numbers of persons aged > or = 65 years during 2010-2030. Worldwide, the average life span is expected to extend another 10 years by 2050. The growing number of older adults increases demands on the public health system and on medical and social services. Chronic diseases, which affect older adults disproportionately, contribute to disability, diminish quality of life, and increased health- and long-term-care costs. Increased life expectancy reflects, in part, the success of public health interventions, but public health programs must now respond to the challenges created by this achievement, including the growing burden of chronic illnesses, injuries, and disabilities and increasing concerns about future caregiving and healthcare costs. This report presents data from the U.S. Bureau of the Census, the World Health Organization, and the United Nations on U.S. and global trends in aging, including demographic and epidemiologic transitions, increasing medical and social costs related to aging, and the implications for public health.