Since 1980, weights in the United States have been increasing alarmingly--and the rest of the world seems to be following suit (see sidebar). Currently, 22.5% of the U.S. population is considered to be clinically obese, as compared to only 14.5% in 1980. What9s more, this "obesity epidemic," as many public health experts call it, affects all demographic groups, including children, and the end does not appear to be in sight. But what9s behind the increase is much less clear; although many researchers blame increased food availability and declining physical activity (see Article on p. 1371), major changes in eating behavior or activity sufficient to account for the recent rapid increase in obesity have not been clearly identified.