This is a review on the effects of physical exercise on weight, body composition and health in prevention and management of obesity. Individuals who maintain or increase their physical activity also show the best weight control over a period of several years. However, physical activity at baseline of prospective studies does not consistently predict weight change, perhaps because of changes in physical activity during the follow-up. Moderate-intensity (e.g. walking) physical activity during diet-induced weight reduction leads to modest improvements in weight, abdominal fat and total fat loss. Moderate exercise 250-300 min/wk is recommended for weight reducing purposes. For maintenance of reduced body weight, more than 60 min moderate exercise per day after weight reduction might be needed. Studies have found a more pronounced training-induced loss of visceral compared to subcutaneous fat in overweight and obese subjects. Physical activity will help to lower risk of insulin resistance, type 2 diabetes, dyslipidemia, hypertension and the metabolic syndrome in overweight and obese individuals and that more pronounced effects can be attained in combination with maintained weight loss. The intensity of physical activity should be at least moderate (>30-40% VO(2)max) to cause metabolic health benefits. A suitable frequency for moderate-intensity physical activity seems to be daily, and for vigorous activity every other day.