Impulse oscillometry (IOS) is a variant of forced oscillation technique described by Dubois 50 years ago, which allows us to measure the reactance of the airways and the resistance of the small and large airways during tidal breathing. It requires minimal patient cooperation from subjects who are unable to perform spirometry, like elders, children and patients with neurologic disorders. IOS can outline the diagnosis of obstructive airway disease, differentiate small airway obstruction from large airway obstruction. It is more sensitive than spirometry for peripheral airway disease in determining the severity of the disease, the exacerbations and evaluate the therapeutic response. Other applications include early evaluation of transplant rejection, cystic fibrosis, vocal cord disorder, bronchiectasis, hypersensitivity pneumonitis, obstructive sleep apnea.