Technical aspects and clinical applications of the two most commonly used noninvasive CO2 monitors, capnography and transcutaneous monitoring, are discussed. Neither accurately reflect PaCO2 in most critically ill patients. However, both monitors give valuable information about other aspects of the patient's physiology. PETCO2 reflects changes in pulmonary perfusion and deadspace ventilation; and, PtcCO2 reflects changes in peripheral perfusion. Thus, both are useful in the critically ill patient, but not necessarily for the assessment of PaCO2.