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Abstract: If it can be confirmed that variations in the level of nitric oxide (NO) in expired air tells us something about the different physiological processes in which this molecule is involved (bronchomotor, vasomotor, immunoregulation), then we will have available a non-invasive technique that would be useful in conditions as varied as asthma, sickle cell anemia, mucoviscidosis and pulmonary arterial hypertension. This technique can be used routinely to the analysis of the concentration of NO in a single expired breath. However, analysis of the concentration of NO in several expired breaths, which is possible from the age of 8–9 years, also makes it possible, using a method that is now standardized, to determine the bronchial production of NO and its alveolar concentration, thus providing a method for differentiating the respective contribution of these two compartments. Other techniques now being developed should soon allow us to calculate even more precisely events during normal ventilation, which will then allow us to make these measurements in babies, in young children, and in patients on artificial ventilation. The aim of this present paper is to discuss the theoretical basis and the techniques of these different approaches. [Copyright &y& Elsevier]