Cerebrospinal fluid formation, composition, circulation, and absorption, and changes in various pathological conditions are discussed briefly in this chapter. Choroids plexus in the ventricles is the main production site of CSF formation (70–80%); the rest is formed by pial surface, cerebral extracellular space, perineural space and the dural nerve sheaths in the spine. Formation is inhibited by enzyme inhibitors (i.e. acetazolamide). Cerebrospinal fluid is considered as plasma ultrafiltrate. Cilia on ependymal cells produce currents that propel CSF towards the 4th ventricle and its foramina into subarachnoid space (SAS). Cerebrospinal fluid passes from the lateral ventricles through interventricular foramina of Munro into the 3rd ventricle and then through the aqueduct of Sylvius into the 4th ventricle. CSF enters into SAS through the paired lateral foramina of Luschka. The respiratory variations and vascular pulsations emanating from the choroid plexus and cerebral arteries causes the ventricles to pulsate providing additional movement to CSF. Cerebrospinal fluid cushions the brain; the blood-CSF barrier functions together with the blood-brain barrier (BBB) and the meninges, to control the internal environment of the brain.