The main objective of the maximal exercise test is to measure a patient's exercise limit and to identify the cause. The test to be interpreted is evaluated first by judging exercise tolerance on the basis of maximal oxygen consumption (VO2max or symptom-limited VO2(VO2 SL). Intolerance is moderate if VO2 is under 85% of the theoretical level and severe when it is under 60%. Interpretation then consists in identifying the cause of the limitation. Ventilatory reserve is the main element for determining whether the limitation is due to ventilatory or cardiac impairment. In case of a ventilatory limitation, ventilatory reserve falls and the ventilatory pattern and gasometric values orient the diagnosis to emphysema, diffuse interstitial lung disease or bronchopneumopathy. If the ventilatory reserve rises, the limitation has cardiac as origin and in this case the oxygen pool is decreased. Persistence of chronotrope reserve can indicate coronary artery disease while the VD/VT ratio is useful for differentiating cardiac and vascular limitations. Peripheral limitations usually produce an early major increase in blood lactate, a high lactate/pyruvate ratio, with lactate or ventilatory thresholds under 40% of theoretical VO2max, especially in case of muscle pain and low watt equivalent. Nevertheless, these parameters cannot be used to confirm a precise diagnosis. The only positive diagnosis given by the exercise test concerns glycogenolysis disorders (por example McArdle's disease) seen as a total absence of increased lactate level. In conclusion, the exercise test is a means of assessing a patient's exercise tolerance and provides arguments for determining the origin of the limitation. However, as no one argument is truly pathognomonic, the diagnosis hypothesis must be based on all the elements available from clinical examination and complementary explorations.