Regular physical exercise is of the utmost importance in the treatment of obesity because exercise is one of the factors determining long-term weight maintenance in weight reduction programmes and because exercise has been associated with a reduced risk for developing type 2 diabetes mellitus and cardiovascular disease. Obesity is associated with an impaired utilization of fat as a fuel during post-absorptive conditions, during beta-adrenergic stimulation and possibly during exercise, although the latter data are controversial. One of the underlying mechanisms for the positive effect of exercise training in obesity may be related to its effects on fat utilization because exercise training has been shown to increase basal fat oxidation and exercise fat oxidation in lean volunteers. Data on the effect of aerobic exercise training on exercise fat oxidation are controversial, whereas the available data indicate that exercise training may not be able to increase resting fat oxidation or 24-hour fat oxidation in obese subjects. Because disturbed muscle fat oxidation may be a primary event in the aetiology of obesity it is of the utmost importance to obtain more information on how and whether exercise training may be able to compensate for these impairments.