Abstract Obesity is due to an imbalance between caloric intake and expenditure. One component of expenditure that has been receiving increasing research attention is exercise. This paper reviews the research on activity level differences in thin and obese persons, and basic research on energy balance, or intake/expenditure relationships. Then meta-analysis techniques were used to study the effects of aerobic walking and jogging exercises for weight and fat loss, the selective effects of exercise for thin and heavy persons, and the parametric effects of increased frequency or expenditure on weight loss. The effects of exercise on weight were reliable, and directly related to expenditure. Assessment of energy balance shows persons do not typically lose as much weight as predicted by exercise, and thin persons show more energy compensation than heavy persons. Comparison of the results of exercise and other forms of weight control suggests the weight losses due to exercise are quite small.