This chapter examines different time- and volume-based capnograms, and analyzes them from a clinical perspective, with a special focus on problems related to ventilation, by far the most common clinical application of capnography. A water trap with a large internal volume can introduce artifacts when high airway pressures during inspiration compress gas in the trap. The capnogram provides evidence of acutely reduced pulmonary perfusion coincident with a drop in cardiac output. The most important use of capnography in the field, in the intensive care unit, and in the operating room comes with the establishment of an artificial airway. An individual tracing of the time-based capnogram left a number of questions unanswered, which the single breath volume-based capnogram provides. The data offered by the volume-based capnogram refine the information offered by time-based capnography.