Impulse oscillometry (IOS) is a variant of forced oscillation technique, described by Dubois over 50 years ago, which permits passive measurement of lung mechanics. In this method, sound waves are superimposed on normal tidal breathing, and the disturbances in flow and pressure caused by the external waves are used to calculate parameters describing the resistance to airflow and reactive parameters that mostly relate to efficient storage and return of energy by the lung. It requires minimal patient cooperation and can be done easily in subjects who are unable to perform spirometry. Importantly, IOS can differentiate small airway obstruction from large airway obstruction and is more sensitive than spirometry for peripheral airway disease. It has been used to study various respiratory disorders, especially asthma and is suitable for measuring bronchodilatory response as well as bronchoprovocation testing. IOS parameters seem to be able to pick up early changes in lung functon such that they are superior to spirometry in predicting loss of control in asthmatic patients and possibly in identifying early airway disease in smokers. Such comparisons, especially for chronic obstructive pulmonary disease, are made difficult by widespread use of spirometric parameters as the diagnostic gold standard. Here, we discuss the principles and technique of IOS and review its application in obstructive airway diseases.