Fractioned exhaled nitric oxide (FeNO) is a non-invasive marker of airway inflammation. While airway functional is easily measurable by tests such as spirometry, airway inflammation in the clinical setting is more difficult to estimate. FeNO attempts to fulfil this unmet need. This chapter discusses the physiological basis of the test in the context of the other currently available tests for airway inflammation, its specific applications in clinical practice and the factors that impact its interpretation. A discussion on nasal nitric oxide testing is also included.