Spirometry is a fundamental test in the diagnosis of lung disease in children, including preschool children. It measures the amount (volume) and speed (flow) of air that can be inhaled and exhaled by the patient. The FVC (forced vital capacity) maneuver is the total amount of air exhaled forcefully following a maximal inhalation, and it is usually the only one performed. The FEV1 (Forced Expiratory Volume in one second) describes the speed (flow) of air exhaled in the first second of the expiratory phase of FVC maneuvers. FVC, FEV1, and the FEV1/FVC ratio are the essential data to evaluate the flow/volume curve of spirometry and differentiate normal, obstructive, restrictive, or mixed (obstructive/restrictive) spirometry patterns. Every anesthesiologist and surgeon must be familiar with spirometry data, essential in ascertaining preoperative lung function and in the follow-up of the patient after the surgical procedure, particularly in the case of thoracic surgery.