Obesity in adolescents has increased by 75% in the past three decades. Cross-sectional and prospective surveys have shown that a large percentage of adolescents, particularly females and even those of normal weight, diet at some time. While moderate changes in diet and exercise have been shown to be safe, significant psychologic and physiologic consequences may occur with extreme or unhealthy dieting practices. Moderate dieting has been shown to be associated with negative self-esteem in some adolescents. The very act of starting any diet increases the risk of eating disorders in adolescent girls. Extreme methods of weight loss can have adverse physiologic effects if not closely monitored. Electrolyte disturbances, cardiac dysrhythmias, and even sudden cardiac death can result from unhealthy or extreme dieting practices. Such practices are associated with other problem behavior in adolescents. We review current information on dieting in teenagers and discuss psychologic and physiologic effects of these practices.