The delivery of O2 and its utilization by tissues requires integration of respiratory, cardiovascular, and microvascular systems. Inadequate tissue oxygenation is a common reason for patients’ admittance to the intensive care unit (ICU). Reversal and prevention of tissue hypoxia are fundamental goals of resuscitation. Thus monitoring of O2 transport and consumption is potentially important for the early reversal and prevention of tissue hypoxia.Multiple organ failure is the most common cause of mortality in the ICU, it is likely that tissue hypoxia plays an important rule in its development.Therefore, maintenance of tissue oxygenation is a major determinant of normal organ function and survival in ICU. Monitoring of O2 transport and consumption has been practiced clinically at the bed side long before the availability of technology for their measurement. It was not until cardiac output and O2 consumption could be measured that the monitoring of O2 transport and consumption became feasible. Monitoring of O2 transport and consumption is now an integral part of clinical practice in many ICUs.