Capnometers measure carbon dioxide (CO2) in expired air and provide clinicians with a noninvasive measure of systemic metabolism, circulation, and ventilation. If two of these systems are held relatively constant, changes in CO2 excretion will reflect the third. CO2 measurement has been advocated as a method of ensuring endotracheal intubation. Because the air in the esophagus has very low levels of CO2, the use of capnometers may help prevent unrecognized esophageal intubation in prehospital and emergency department settings. In patients with normal perfusion and ventilation, end-tidal CO2 measurements closely reflect alveolar PCO2. Capnometry may decrease the need for frequent arterial blood gas measurements. A sudden change in end-tidal CO2 measurement may indicate decreased lung perfusion and an early shock state. Thus, capnometry may be useful to monitor critical patients in the ED. In addition, capnometry has potential usefulness as a noninvasive indicator of the efficacy of ongoing CPR efforts. End-tidal CO2 has been shown to correlate with cardiac output, perfusion pressures, and successful resuscitation in experimental models of cardiac arrest. Further clinical studies are needed to define the role of capnometry in emergency medicine.