Anesthesiologists monitor their patients' breathing by listening to the lungs or auscultating over the trachea, counting the respiratory rate, watching chest movement and tidal volume, and employing pulse oximetry and capnography. This chapter focuses on issues related to capnography specific to anesthesia and the operating room. Capnography is the best monitor to identify complete disconnection of the breathing circuit. Exhaled tidal volume is a sensitive indicator of leaks and partial disconnects during mechanical ventilation. Capnography will continue to detect expired CO2 as long as the patient's exhaled tidal volume passes sidestream or mainstream sampling ports. Patients with chronic obstructive pulmonary disease or asthma exhibit typical capnograms with upsloping expired values brought about by the slow emptying of partially obstructed segments of the lungs. Intermittent PaCO2 determination has been used as a routine parameter for acid-base management during cardiopulmonary bypass (CPB).