Transcutaneous PCO2 electrodes are noninvasive, reliable, and accurate indicators of the trend in arterial PCO2. Their combination with occasional arterial sampling allows them to be calibrated in vivo for use in a wide range of patients, even in the presence of a severely compromised haemodynamic state. Their role in the ITU as monitors of continuous ventilation and during weaning has been established. However, their potential for use during anaesthesia, acute asthma, exacerbations of chronic airflow limitation, and in studies into the normal and abnormal mechanisms of ventilation during sleep has yet to be realized.