Asthma is a respiratory disease affecting more than 300 million people around the world. Airflow obstruction and inflammation due to asthma usually involve large airways, but recently small airway involvement (internal diameter < 2 mm) has been shown to represent one of the main determinants of asthma and asthma control. In fact, compared to large airway involvement, small airway dysfunction (SAD) has been demonstrated across all the asthma severity in the majority of patients, as assessed with Global Initiative for Asthma (GINA) steps. Clinically, SAD is associated with, among other features, exercise-induced bronchoconstriction, asthma-related night awakenings, obesity/overweight, more severe airway hyperresponsiveness, worse asthma control, and more severe exacerbations. Impulse oscillometry (IOS), a forced oscillation technique (FOT) requiring less effort than spirometry from the patients, demonstrated to accurately measure SAD in children and adults. The fall in resistance from 5 Hz to 20 Hz (R5–R20), which is the most used index for the resistance of peripheral airways, is how SAD is usually identified by IOS. Other crucial parameters measured by IOS are the reactance at 5 Hz (X5), reflecting elastic recoil of the peripheral airways, the resonant frequency (Fres), which is the frequency at which the inertial properties of the airway and the capacitance of the lung periphery are equal, and the reactance area (AX), reflecting the elastic properties of the lung periphery. In this mini review, the latest findings on the utility of IOS to identify SAD and the associations between SAD and clinical features in adult asthmatic patients were addressed.