Posttraumatic increase of intracranial pressure (ICP) is a strong prognostic factor for the outcome of patients after traumatic brain injury. After exhausting all compensatory mechanisms ICP increases exponentially, where ICP(norm)=(CSF production*CSF flow resistance)+venous pressure((sinus sagittalis))=10-15 mmHg. The ICP curve is influenced by the compliance (DeltaV/DeltaP) and elasticity (DeltaP/DeltaV) of the brain. Marmarou could demonstrate that the non-linear cranio-spinal pressure-volume relationship describes a logarithmic, mono-exponential, strongly linear relationship between pressure and volume and named this the pressure volume index (PVI=log ICP/DeltaV). The pressure volume index describes the volume necessary to increase ICP by a factor of 10. Additionally to PVI the measurement of volume-pressure response (VPR) was introduced. The continuous intracranial compliance could be determined on the principle of pulsatile volume increases as an equivalent of very small intra-cranial volume increases. However, to ascertain functional status of the injured brain a combination of measurements of different parameters, such as tissue oxygen partial pressure (p(ti)O2), cerebral blood flow (CBF), microdialysis and electrocorticography (ECoG) is recommended.