Knowledge of normal and impaired pulmonary gas exchange is essential to the anaesthesiologist. Analysis of an arterial blood sample allows evaluation of whether or not pulmonary gas exchange is normal. For this purpose comparison with the oxygenation index or the alveolar-arterial PO2 difference is helpful. Pathological changes of these variables are mainly caused by ventilation-perfusion (VA/Q) mismatch. In daily practice, venous admixture or intrapulmonary shunt can be calculated using arterial and mixed-venous blood. By analysing arterial and expired PCO2, dead-space ventilation can be determined, but extended analyses of VA/Q distribution are not possible in daily practice. However, knowledge of the principles of typical disturbances of pulmonary gas exchange in acute and chronic lung disease allows the use of therapeutic strategies based on the pathophysiological changes.