Cerebral microdialysis is one of the latest neuromonitoring modalities introduced to clinical practice. It is a bedside monitor used to assess brain tissue biochemistry. The principle of this technique is closely related to brain metabolism and dialysis. Microdialysis helps monitoring different metabolites related to energy and metabolic cascades (glucose, lactate and pyruvate), amino acids (glutamate) and markers of cell membrane degradation (glycerol). The principle of CMD is to mimic a capillary blood vessel in the brain for the assessment of local cerebral metabolism . A tubular semi-permeable membrane on the tip of the CMD catheter is perfused with an isotonic or colloid-enriched fluid that equilibrates with the extracellular space by simple diffusion. All molecules small enough to pass the membrane follow their electro-chemical gradient into the tube. Established catheters have a membrane length of 1 cm and pore sizes of either 20 or 100 kDa, which do not show differences in the recovery of small molecules . A perfusion speed of 0.3 µl/min is recommended in clinical use, which leads to a relative recovery rate (dialyzate concentration divided by true concentration) of about 70% for the most commonly assessed molecules. Criteria for CMD monitoring are not well defined. It can be used as part of the “multimodal neuromonitoring bundle” in ventilated (“poor-grade”) patients or in patients with a secondary neurological deterioration. As a primary monitoring device, the probe should be placed in the frontal lobe (anterior/middle cerebral artery watershed), ipsilateral to the ruptured aneurysm, or the maximal blood clot load. When used in patients with secondary deterioration, location can also be guided by local practice to identify tissue at risk