Cerebrospinal fluid (CSF) is produced by the choroid plexus in the lateral, third, and fourth ventricles at a rate of approximately 500 mL/day, with a total volume of 150 mL at any time. CSF flows through the ventricular system, exits through the foramina of Luschka and Magendie into the subarachnoid space, and is reabsorbed by arachnoid granulations into the dural venous sinuses. Lumbar puncture (LP) accesses the subarachnoid space between L3-L4 or L4-L5 (below the conus medullaris at L1-L2 to avoid spinal cord injury). Opening pressure reflects intracranial pressure (normal 10-20 cmH2O). CSF analysis provides critical diagnostic information: elevated WBC (pleocytosis) indicates meningitis, elevated protein suggests inflammation or obstruction, decreased glucose indicates bacterial consumption, and xanthochromia (yellow discoloration from bilirubin) confirms subarachnoid hemorrhage. Traumatic tap (blood from needle trauma) clears with sequential tube collection, while true SAH produces uniform blood and xanthochromia.