Since gas exchange is a primordial function of the lungs and the conductive airways, respiratory assessment is of paramount importance. Capnography has been utilized in surgical patients for over three decades to confirm tracheal intubation and assess ventilation. Nitrogen washout provides an estimate of functional residual capacity, total lung volume, deadspace volume, and alveolar volume. Clinicians typically utilize exhaled CO2 concentration against time during a respiratory cycle. A number of applications are available in and out of the operating room. Capnography can be used as a continuous monitor of alveolar ventilation in patients with lung disease or hemodynamic instability. Mainstream capnometry appears to provide more accurate PETCO2 than conventional sidestream capnometry during spontaneous breathing in non-intubated patients. In the opinion of some investigators, the technology should be employed in all cases requiring sedation in or out of the operating room.