Asthma is a chronic inflammatory disorder of the airways that is characterized by episodic symptoms. In this regard, asthma management has classically involved periodic re-assessment by the health-care provider, during which therapy is altered mainly based on clinical and physiological parameters, such as assessment of symptoms, spirometry and peak expiratory flow monitoring. In this context, various markers of airway inflammation (e.g. eosinophils in the induced sputum, nitric oxide in the exhaled air) have been proposed to assess the severity of asthma and to adjust the therapy accordingly. The evaluation of airway hyper-responsiveness with different stimuli has also been suggested as a new tool to monitor asthma. However, the lack of definite relationships between airway inflammation and asthmatic symptoms strongly limit the use of markers of asthma severity in the clinical setting. Therefore, the need of new tools to assess the severity of asthma is raised. The ideal measurement employed to establish the proper asthmatic therapy should be safe, non-invasive, easy to perform, reproducible and accurate, and have the capability to monitor the changes induced by the therapeutic interventions. A careful review of the available techniques, and the evaluation of their sensitivity and specificity in the clinical setting is warranted.