Spirometry is one of the functional tests most used in respiratory medicine to assess lung function in health and disease conditions.Its success is grounded on solid principles of lung mechanics that state that maximal flow on expiration is limited by the physical properties of airways and lung parenchyma.In contrast,on inspiration,flow depends on the force generated by the inspiratory muscles.Reduced expiratory forced flow and volumes usually reflect a deviation from health conditions.Yet due to a complex interplay of different obstructive and restrictive lung diseases within the multiple structural dimensions of the respiratory system,flows and volumes do not always perfectly reflect the impact of the disease on lung function.The present review is intended to shed light on a series of artefacts and biological phenomena that may confound the clinical interpretation of the main spirometric measurements.Among them is thoracic gas compression volume,the volume and time history of the inspiratory manoeuvre that precedes the forced expiration,the effects of heterogeneous distribution of the disease across the respiratory system,and the changes in lung elastic recoil. Spirometry is one of the functional tests most used in respiratory medicine to assess lung function in health and disease conditions. Its success is grounded on solid principles of lung mechanics that state that maximal flow on expiration is limited by the physical properties of airways and lung parenchyma . In contrast, on inspiration, flow depends on the force generated by the inspiratory muscles. Reduced expiratory forced flow and volumes usually reflect a deviation from health conditions. Yet due to a complex interplay of different obstructive and restrictive lung diseases within the multiple structural dimensions of the respiratory system, flows and volumes do not always complete reflect the impact of the disease on lung function. The present review is intended to shed light on a series of artefacts and biological phenomena that may confound the clinical interpretation of the main spirometric measurements. Among them is thoracic gas compression volume, the volume and time history of the ins piratory manoeuvre that precedes the forced expiration, the effects of heterogeneous distribution of the disease across the respiratory system, and the changes in lung elastic recoil.