During the past 10 years, instrumentation has been developed that can continuously and noninvasively measure changes in carbon dioxide and oxygen. The information generated, which cannot be obtained through the human senses, provides vital clinical data regarding the effectiveness of intubation, ventilation, circulation, oxygenation, and the circuit. This instrumentation plays a major role in decision making both in the safe conduct of anesthesia and mechanical ventilation as well as in the detection and prevention of potentially catastrophic mishaps. For these reasons, a review of what has been learned regarding the instrumentation, collection, and interpretation of the clinical data, and the clinical value of the information is timely. The clinical significance of the carbon dioxide and oxygen waveforms, inspired to expired carbon dioxide and oxygen differences, alveolar-arterial gradients, and global supply-to-demand oxygen relationships measured by capnography, oxygraphy, and pulse oximetry are addressed in this essay.