The FOT is a non-invasive method for assessing respiratory mechanics. It enables the measurement of respiratory system impedance by applying pressure oscillations through a loudspeaker at the subject mouth and then studying its deformation, which is commensurate to the resistance opposed by the respiratory system. The main parameters which can be determined with the FOT are the impedance (Z) and the components of respiratory resistance (Rrs) and reactance (Xrs). The FOT has been predominantly applied to the study of respiratory mechanics for research purposes; however, preclinical experiments and subsequently observational clinical studies have demonstrated that FOT can effectively assess airway obstruction, bronchodilator response, bronchial hyperresponsiveness, and the presence of small airways disease. More recently, studies on the FOT in restrictive lung diseases have also been reported. Nonetheless, international guidelines on the precise applications of the FOT in lung diseases are still lacking. The aim of the review was to describe the technical aspects related to the FOT methodology in clinical practice and to provide a concise state of the art on the applications of the FOT in obstructive and restrictive lung diseases.