Global cerebral oxygenation can be measured by means of a catheter introduced in the internal jugular vein and placed retrograde in the jugular bulb. The measure of oxygen saturation sampled from the jugular vein (SjvO2) depends on cerebral metabolism and blood flow. This parameter describes the relative balance between oxygen delivery to the brain and oxygen consumption by the brain. SjvO2 remains normal until cerebral blood flow is proportional to cerebral metabolic demands. Any disturbances that increase cerebral metabolism and/or diminishes cerebral oxygen supply determines a reduction of SjvO2. Correspondingly, a decrease of oxygen consumption and/or an increase of oxygen supply may induce an increase of SjvO2. Therefore, SjvO2 is a useful monitor to assess the adequacy of cerebral circulation in patients with neurologic illness, allowing detection of state of hypoperfusion. Monitoring cerebral oximetry in comatose patients is of great importance in order to prevent, detect, control and understand secondary brain insults and damage which are mainly ischemic/hypoxic in nature. Although SjvO2 was shown to be highly sensitive in the presence of global hypoxia or ischemia, the occurrence of focal ischemia may still go undetected. Besides this, elevated SjvO2 should not be universally interpreted as hyperaemia. Instead, the presence of an elevated SjvO2 is a heterogeneous condition. Increased SjvO2 may be alarming prognostic indicators that carry important implications for comatose patients management.