Disturbances of the oxygenation of a patient during the early postoperative period occur frequently. Their mechanisms are complex. Alveolar related hypoxemia can be distinguished from pulmonary or nonrespiratory related hypoxemias. Those of alveolar origin are characterized by a drop in the alveolar oxygen pressure. Those of pulmonary origin arise mainly due to ventilation perfusion distribution problems or an increase in the intrapulmonary shunt. Those of nonrespiratory origin are mainly caused by a drop in cardiac output or by an increase in the tissular oxygen consumption. Patients showing signs of postoperative oxygenation trouble usually recover rapidly. Severe cases require intensive care treatment.