This chapter provides detail about the monitoring of the pulmonary functions. Postoperative pulmonary complications (PPCs) occur in 5–10% of patients undergoing nonthoracic surgery and in 22% of high risk patients. Postoperative pulmonary complications are the second most leading cause of major morbidity and mortality following surgery. Monitor of pulmonary functions in the perioperative period in patients at risk and those with respiratory diseases with thorough history, physical exam supported with investigations, imaging, and pulmonary function tests in carefully selected patients help in early identification of such patients and initiate appropriate measures to mitigate the risks. Pulmonary function tests are tests conducted using special equipments to assess lung functions in terms of respiratory mechanics, gas exchange, and cardiopulmonary interaction. General indications for performing pulmonary functions tests, perioperative indications of pulmonary function tests, brief insight into pulmonary functions tests, assessment of respiratory muscle strength, tests for cardiopulmonary interaction, etc. are some other important topics also covered in this chapter.