National survey data from the U.S. show that the prevalence of overweight and obesity among adults remained relatively constant over the 20-year period from 1960 to 1980, began to increase around the mid-1980s and has continued to increase. Data for children and adolescents, based on different definitions, show the same pattern. It can sometimes be more useful to look at the whole distribution of body mass index, rather than on prevalence estimates based on pre-defined cutoffs. Data from several countries suggest that for both adults and children, the distribution of BMI has become more skewed over time. Although many hypotheses have been put forward about the causes of the increases, data to address these issues are sparse. Obesity is a well-known risk factor for numerous health conditions. Nonetheless, the health consequences of the increases in obesity have not been fully delineated. Increases in diabetes have been noted in conjunction with the rise in obesity. On the other hand, declines in some other cardiovascular risk factors have been seen at all BMI levels. Rising life expectancy and decreasing heart disease mortality rates seem to confound some of the expectations about the effects of increasing obesity on mortality. The effects of obesity on morbidity may be greater than its effects on mortality. The increasing prevalence of obesity poses challenges for researchers and for policy makers.