Monitoring cerebral oxygenation is receiving increased attention as we recognise more that the brain’s high oxygen demand and sensitivity to anaesthetic agents, as well as surgical and critical clinical events, make it particularly vulnerable to hypoxaemia. Near-infrared spectroscopy (NIRS) enables real-time non-invasive measurement of brain tissue oxygen saturation (rSO2), which can be useful in a variety of settings, including but not limited to surgery, cardiac arrest and septic shock. NIRS can help in the detection of cerebral hypoxic events, which is crucial for the prevention of primary and secondary brain injury, especially in clinical scenarios where invasive neuro-monitoring is not deemed suitable or available. Monitoring cerebral oxygenation allows the detection of desaturation episodes that escape standard haemodynamic monitoring, therefore potentially preventing organ dysfunction and damage. However, clear threshold values for the detection of brain ischaemia are not yet fully defined, and therefore NIRS is best suited for trend monitoring. The integration of NIRS in clinical practice has grown considerably in recent years and is now part of multimodal set-up in the operating theatres and numerous intensive cares.