Capnography has made steady inroads in the ICU and is increasingly used for all patients who are mechanically ventilated. There is growing recognition that capnography is rich in information about lung and circulatory physiology and provides insight into many diseases and treatments. These include conditions of impaired matching of ventilation and perfusion, such as pulmonary embolism and obstructive lung diseases; circulatory questions, such as the adequacy of chest compressions during cardiac arrest or fluid responsiveness in patients in shock; and the safety of procedural sedation. In this review, we emphasize analysis of the entire capnographic waveform as a way to glean additional useful information. We also discuss important limitations of capnography, especially when it is considered to be a surrogate for Paco2.