Impulse oscillometry is a noninvasive procedure that can be performed within few minutes. The purpose of the procedure is to measure the resistance of the small and large airways, as well as the reactants of the airways. It is gradually gaining popularity in evaluating lung function, particularly in patients with asthma and COPD. In contrast to spirometry, the test performs measurement during tidal breathing. In other words, forced exhalation is not required. Other advantages include, but are not limited to, evaluating COPD patients' reversibility which is rarely noted on spirometry. IOS also is tool for chronic management of patients with asthma and COPD while on treatment. It can evaluate children with asthma even as young as 2 years old. Spirometry requires the child to cooperate and usually is of meaningful use beginning at the age of 5 years old. Other potential applications include early evaluation of transplant rejection, cystic fibrosis, and vocal cord disorder. In this chapter, we will explore the procedure itself, the settings, advantages and disadvantages, and comparative data with spirometry.