Until gene therapy allows to detect and manipulate those genes involved in excessive accumulation of fat in children and adolescents, prevention seems to be the only realistic solution for the increasing prevalence of childhood obesity. Traditional approaches include energy intake restrictions and some extra energy expenditure. However, this restricting energy intake might not be the best option for growing youths who need to be in a positive energy balance in order to ingest sufficient amounts of the nutrients needed for optimal growth. Physical inactivity is worldwide one of the biggest problems of public health. In Spain, approximately 66% of boys and girls under 15 years do not perform or rarely perform any kind of physical activity during their leisure free time. Vigorous physical activity (VPA), without restriction of energy intake, can produce favourable effects on body composition (reduction of total and visceral fat mass, and increased bone mass...), fitness levels and other cardio-metabolic risk factors. Moreover, VPA is often accompanied by increases, not decreases, in dietary energy intake. It appears that exercise doses of 155-180 minutes per week at moderate to high-intensity are effective in improving the body composition and fitness of overweight youths. For youths who are not overweight prior to the intervention, larger doses of approximately 300 minutes per week may be needed to prevent excess accretion of fat. Data presented in this review suggests that adding 2-3 hours per week of sport activities to the compulsory physical education courses seems to be effective in preventing excessive fat mass accumulation at the same time that it facilitates lean mass accretion and improves physical fitness in growing youths. The battle against childhood obesity should focus on changing the lifestyles of children and youths. Lifestyle interventions are more likely to be successful if they emphasize reduction of time devoted to sedentary activities and increased participation in daily VPA, rather than restriction of dietary energy intake.