Children spend a substantial portion of their lives watching television. Investigators have hypothesized that television viewing causes obesity by one or more of three mechanisms: (1) displacement of physical activity, (2) increased calorie consumption while watching or caused by the effects of advertising, and (3) reduced resting metabolism. The relationship between television viewing and obesity has been examined in a relatively large number of cross-sectional epidemiologic studies but few longitudinal studies. Many of these studies have found relatively weak, positive associations, but others have found no associations or mixed results; however, the weak and variable associations found in these studies may be the result of limitations in measurement. Several experimental studies of reducing television viewing recently have been completed. Most of these studies have not tested directly the effects of reducing television viewing behaviors alone, but their results support the suggestion that reducing television viewing may help to reduce the risk for obesity or help promote weight loss in obese children. Finally, one school-based, experimental study was designed specifically to test directly the causal relationship between television viewing behaviors and body fatness. The results of this randomized, controlled trial provide evidence that television viewing is a cause of increased body fatness and that reducing television viewing is a promising strategy for preventing childhood obesity.