A major responsibility of the critical care practitioner is to assure adequate ventilation of the critically ill patient. The traditionally used methods for evaluating ventilation, such as physical examination and measurement of vital signs, are indirect. The most commonly used direct method, measurement of arterial carbon dioxide tension, is invasive and intermittent. Capnography provides the critical care practitioner with a continuous, noninvasive, and accurate assessment of ventilation. To interpret capnographic data, the practitioner must have a clear understanding of normal and abnormal patterns of carbon dioxide elimination in the lung. We review relevant respiratory physiology as a basis for understanding the value of capnography. The technology on which capnography is based is described with emphasis on methods of gas sampling, limitations of capnography, and features available on currently marketed instruments. Representative capnograms are presented and the data interpreted to enable the practitioner to determine when capnography is an appropriate monitor for the critically ill adult.