Objective: The physiological background for intracranial pressure, ICP, increase at brain compression is presently unknown. Current knowledge of the relationship between ICP and volume of brain compression is based on animal experiments, which has led to the theory of elastic brain tissue compression, which like a loaded spring causes the ICP increase. However, tests of brain tissue composition, or compression, find no basis for elasticity, or compression. The aim of this article is to describe the physiological features controlling ICP since it is necessary for the understanding of intracranial physiology, but also since ICP is an important parameter in neurointensive care medicine. Methods: This evaluation is based on the physiological consequences of the closed intracranial compartment, the thin-walled, compressible venous bed, the auto-regulation of cerebral blood flow, CBF, to blood pressure changes, and the coupling between ICP and the intra-capillary blood pressure through the thin, flexible capillary wall. These features influence the intracranial venous blood volume, the venous vascular resistance, and the CBF. Together these features are shown to revile the ICP dependence on the volume of brain volume compression. Results: The evaluation leads to a formula that describes the relationship between ICP and changes in the brain volume. The formula predicts an exponential ICP increase at brain compression like in animal experiments but, contrary to the elastic tissue theory, a gradually higher exponential for larger compression. Conclusion: The found relationship between ICP and changes in brain volume may prove to be a useful tool in neuro-intensive care patients for following changes in intracranial volumes by the aid of continuous ICP measurements. Since brain-compressing volumes includes the arterial pulse volume, the ICP formula may also enable CBF calculation from ICP measurements. The physiological relationships leading to the description of the relationship between ICP and brain volume change may by themselves help in understanding other intracranial phenomena, like ICP plateau-waves.