Obesity has reached epidemic proportions in the United States, but there are few proven strategies for either preventing further weight gain or producing permanent weight loss. Our first priority should be to prevent the gradual weight gain experienced by much of the population. Although this will require less behavior change than producing and maintaining weight loss, helping Americans make and sustain the behavior changes needed to prevent gradual weight increases will be challenging. Because approximately 65% of Americans are already overweight or obese, we must also develop effective strategies to help achieve and maintain an amount of weight loss that improves their health and quality of life. Our real challenge is not in helping people lose weight but in helping them keep it off. Many programs have been shown to produce weight loss but few, if any, have been successful in maintenance of weight loss. Our challenge is in understanding how to help people keep off the weight they can lose in several ways.