Exhaled nitric oxide fraction (FENO) values can be easily measured using portable analysers and are a surrogate marker of airway eosinophilia. FENO may be useful in diagnosing and monitoring conditions characterised by airway eosinophilia, i.e. asthma and possibly COPD. Many factors other than asthma and COPD affect FENO, especially atopy, which is associated with elevated FENO. One guideline recommends that FENO should be used as part of the diagnostic pathway for asthma diagnosis in adults and children aged >5 years. The role of FENO in monitoring asthma is even less clear, and most guidelines do not recommend its use outside of specialist asthma clinics. Currently, FENO is not recommended for diagnosis or monitoring of COPD. Although FENO is starting to find a place in the management of asthma in children and adults, considerably more research is required before the potential of FENO as an objective measurement in asthma and COPD can be realised.