Asthma is a chronic respiratory condition affecting many adults and children. An association between asthma and raised gaseous nitric oxide in the exhaled breath was first reported in 1993. In the absence of an objective test for diagnosing and monitoring asthma, a vigorous research effort has subsequently sought to determine the nature of the relationship between exhaled nitric oxide (ENO) and asthma and to determine the clinical utility of ENO measurements in asthmatics. Many methodological issues relating to ENO measurements have also been addressed. The evidence indicates that ENO originates from a number of sources and the “excess” ENO in asthmatics is mostly derived from the lower respiratory epithelium where ENO probably reflects eosinophilic airway inflammation. Clinical studies have revealed that ENO may be useful in the diagnosis of atopic asthma but not non-atopic asthma. Longitudinal measurements of ENO have been found to correlate with asthma symptoms, including relapse of symptoms after cessation of corticosteroids treatment. In summary, over a relatively short time ENO has become recognised as a useful objective tool for diagnosing and monitoring asthma. More clinical studies will be required in order for ENO to become fully established in the diagnosis and management of asthma.