Cerebrospinal fluid (CSF) is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). It acts as a "cushion" or buffer for the cortex.

Cerebrospinal fluid also occupies the ventricular system of the brain and the spinal cord. It is mainly produced by the choroid plexus, but also by the ependymal lining of the brain's ventricles. The CSF is formed by the choroid plexus of the ventricles circulates through the interventricular foramina into the third ventricle and then via the mesencephalic duct (cerebral aqueduct) into the fourth ventricle. From there, the fluid passes to the subarachnoid space through two lateral apertures and one median aperature and is then absorbed by the venous system to the blood circulation.

The total amount of cerebrospinal fluid is about 150 ml, and about 500 ml is produced every day, which indicates its very active circulation. Cerebrospinal fluid can be tested for the diagnosis of a variety of neurological diseases. Usually, it is obtained by a procedure called lumbar puncture in an attempt to count the cells in the fluid and to detect the levels of protein and glucose. These parameters alone may be extremely beneficial in the diagnosis of central nervous system infections (especially meningitis and subarachnoid hemorrhage). Moreover, a cerebrospinal fluid culture examination may yield the microorganism that has caused the infection. By using more sophisticated methods, such as the detection of the oligoclonal bands, an ongoing inflammatory condition (for example, multiple sclerosis) can be recognized.

Lumbar puncture can also be performed to measure the intracranial pressure, which is increased in a condition known as hydrocephalus, or in some forms of head trauma (this is rarely due to an increase in cerebrospinal fluid itself; however, removing some of the fluid for the puncture can be temporarily helpful sometimes).