Microdialysis is an invasive technique based on placement of a small catheter into the brain parenchyma. The distal end of the catheter is a dialysis membrane. The lumen is perfused with Ringer acetate into which molecules are taken up by osmosis from the surrounding tissue. The irrigation fluid can be analysed online bedside (most commonly) or later at some convenient time. With the current technique, approximately 70% of the original parenchymal concentrations of the most commonly analysed substrates can be recovered in this fluid. By positioning the catheter in an area of interest, e.g. a contused area, the method can give consecutive samples of the extracellular fluid, thereby monitoring the ongoing metabolism of this tissue. The normally analysed substrates bedside is glucose, lactate, pyruvate, glycerol, and glutamate. It is recommended to use intracerebral microdialysis as a complement to intracranial pressure (ICP) and cerebral perfusion pressure (CPP) monitoring in severe cases of TBI subarachnoid haemorrhage and TBI. The lactate/pyruvate ratio is considered to be a sensitive marker for the development of ischemia, whereas glycerol is considered to reflect cell death.