This article is an academic review of the application in children of the measurement of fractional exhaled nitric oxide (FENO). We outline the joint American Thoracic Society/European Respiratory Society recommendations for online measurement of FENO in both cooperating children and children unable to cooperate, offline measurement with uncontrolled exhalation flow rate, offline measurement with controlled exhalation flow rate using a dynamic flow restrictor, and offline measurement during tidal breathing in children unable to cooperate. This is followed by a review of the normal range of values for single-breath online measurements obtained with a chemiluminescence FENO analyzer (geometric mean, 9.7 parts per billion [ppb]; upper limit of the 95% confidence interval, 25.2 ppb). FENO values above 17 ppb have a sensitivity of 81% and a specificity of 80% for predicting asthma of an eosinophilic phenotype. We discuss the response of FENO values to anti-inflammatory treatment and the use of this marker in the management of asthma. Results obtained with chemiluminescence and portable electrochemical analyzers are compared. The portable devices offer the possibility--in children over 5 years of age--of accurate and universal monitoring of exhaled nitric oxide concentrations, an emerging marker of eosinophilic inflammation in asthma that facilitates diagnosis, monitoring of disease progression, and assessment of response to therapy.