Obesity has a close connection with energy balance. Available methods of measurement are, however, too coarse to measure minor daily oscillations that over the months or years can indicate a rise of several kilograms. Basal metabolism is an important part of total energy consumption. This varies with age, sex and body composition. Several investigations appear to show that physical activity can raise basal metabolism. With the help of direct and indirect calorimetry it has been possible to calculate energy consumption above the basal figure. However, in calculating the energy gain through exercise it is necessary to subtract the energy consumption that would otherwise have taken place. Theories have been put forward that the different types of overweight (hyperplastic and hypertrophic) show different gains from physical training as regards weight reduction. However, classification of overweight persons in groups is somewhat uncertain. Physical activity has been found to restore to normal the metabolic changes which are often associated with obesity. There are also cases in which no loss of weight occurs. Physical training has a favourable effect in regulating appetite. Dropout from training programmes is considerable in the overweight. Associated locomotor symptoms may also complicate training. Even though there is little evidence in the literature to show that the majority of overweight persons lose significant amounts of weight without accompanying dietic restrictions, physical training is undoubtedly of great value in these groups.