It is important to keep in mind the differences between PETCO2, alveolar CO2, and arterial PCO2 (PaCO2) as extremes of temperature and altitude, and the potential for sensor interference by condensation or various body fluids, may significantly affect the performance of these devices. This chapter presents the evidence for use of PETCO2 monitoring to guide ventilation in the field and reviews each type of device available, discussing the advantages and disadvantages of each. In theory, monitoring of PETCO2 data should lead to a low incidence of hyperventilation, regardless of whether manual or mechanical ventilation is used. Quantitative capnometry has great potential for guiding ventilation in the prehospital arena. Advances in the technology for PETCO2 monitoring, including capnometry and capnography, have allowed these devices to be small and durable enough to be carried into the field, where they can help avoid hyperventilation and injurious ventilation patterns.