SummaryThe best available assessment of aerobic capacity is measurement of the consumption of oxygen at maximal dynamic effort. When carefully administered, this measurement is remarkably reproducible. Major sources of error in obtaining this measurement include improper gas collection apparatus, inaccurate flow meters, and failure to correct for water pressure in the expired gas. Obtaining a truly maximal effort in an objective manner is also a major limitation. The protocol used in the measurement is of minor importance although treadmill testing usually gives higher values than stationary bicycling.The cost and inconvenience of direct methods of measurement have made indirect methods attractive; however, these methods give less accurate quantifications of aerobic capacity and are not as useful to the athlete interested in gauging his or her improvement following a chronic exercise programme.Extrapolating data obtained from the laboratory to performance in specific athletic endeavours is tenuous. Of course, elite marathon runners will have high maximal oxygen consumption. But one cannot accurately predict how individual athletes will perform in their events; these measurements do not account for the psychological component so important in athletic performance.Perhaps the best use of the measurement of maximal oxygen consumption in athletes is in assessing the success of training programmes in a longitudinal manner.