Pulmonary insufficiency is the most common complication after surgical procedures. From minor atelectasis to acute respiratory distress syndrome (ARDS), postoperative pulmonary insufficiency occurs in up to 50% of surgeries. Here we discuss the anatomy, mechanics, and pathophysiology of pulmonary insufficiency; preoperative and postoperative assessment of lung function; and treatment for pulmonary insufficiency with regard to atelectasis, pulmonary edema, and ARDS. Pulmonary insufficiency secondary to cardiac disease, thromboembolism, and central nervous system depression are discussed elsewhere. Preoperative identification of risk factors for pulmonary insufficiency and understanding the physiologic changes in the perioperative period can potentially mitigate postoperative respiratory failure.