A system dynamics simulation model was developed for understanding trends in obesity in the United States. Data on population body weight from 1971-2002 were combined with information from nutritional science and demography into a single analytic environment for conducting simulated policy experiments. Interventions among school-aged youth and others were simulated to learn how effective new interventions would have to be to alter obesity trends; which population subsets ought to be targeted; and how long it takes for those actions to generate visible effects. One finding is that an inflection point in the growth of overweight and obesity prevalences probably occurred during the 1990s. Another is that new interventions to assure caloric balance among school-age children—even if very effective—would likely have only a relatively small impact on the problem of adult obesity. More comprehensive efforts at all ages are needed to avoid the high costs and heavy burden of disease due to adult obesity.