Oxygen delivery (DO2) is the product of cardiac output and arterial oxygen content. The 'critical' level of oxygen delivery is the value below which DO2 fails to satisfy the metabolic demand for oxygen. This causes a failure of cellular energy production defining a state of shock. The clinicians goal is to ensure that oxygen delivery exceeds the oxygen extraction at the tissue level. These physiologic principles and oxygen supply dependency will be highlighted in this presentation to allow the veterinarian to use these principles to make clinical decisions for their critically ill patient.