This chapter reviews the background and theory of complete and partial CO2 rebreathing Fick cardiac output (QT) measurement, the literature on clinical testing, and presents examples that demonstrate its utility during acute hemodynamic challenges. The classic Fick principle was designed to measure pulmonary capillary blood flow (QC), which comprises 98% of QT in subjects with little or no intrapulmonary or cardiac shunting. Clinical experience has repeatedly demonstrated that severe acute reduction in pulmonary blood flow during constant ventilation, e.g. due to ventricular fibrillation, is accompanied by a major PETCO2 reduction. The most critical need for continuous QT measurements is seen in patients with hemodynamic instability. Clinical experience with non-invasive cardiac output monitoring has also provided readily recognizable hemodynamic profiles of vasodilation, hypovolemia, sepsis, and acute heart failure. Continuous non-invasive QT monitoring can provide critical cardiovascular information during everyday clinical practice.