Exhaled nitric oxide (NO) analysis has taken an increasing importance in diagnosing allergic asthma and monitoring its treatment. Several studies have shown a good correlation with different parameters of eosinophilic inflammation in atopic asthmatics, whereas the correlation with lung function parameters is rather poor. However, multiple factors influence exhaled nitric oxide concentrations such as viral infection, allergen exposure, air pollution, tobacco smoke exposure or spirometry. Nevertheless, several scenarios could be identified in recent years where exhaled NO might add to the information derived from conventional assessments such as pulmonary function testing. These include monitoring steroid therapy and therapy compliance in allergic asthmatics, and assessing the risk of asthma exacerbation after interruption of steroid therapy. Exhaled NO analysis has also proven helpful in the diagnostic evaluation of patients with asthma-like symptoms. For instance, nasal NO concentration is very low in patients with primary ciliary dyskinesia. A further possible field of application might become the analysis of infants and toddlers, however, more methodological studies are needed for a standardised and reproducible measurement procedure. Hundreds of papers have been published on exhaled NO. Nonetheless, many questions remain open, especially concerning interindividual variation of NO concentrations and the relevance of exhaled NO analysis for the optimization of long-term asthma therapy.