Fractional exhaled nitric oxide (FeNO) is a biomarker for type 2 inflammation in the airways, and its measurement has the distinct strengths of being simple, rapid, convenient, and reproducible. Since the discovery of FeNO in the 1990s, its measurement tools have been extensively developed and validated by several researchers. In particular, FeNO showed the potential to support the diagnosis of asthma and eosinophilic bronchitis. Since it has begun to be widely utilized in clinical research areas as well as daily practice, we summarized the standardized diagnostic procedures and suggested the clinical application of FeNO measurement in Korean population.