This chapter discusses cerebrospinal fluid (CSF), which is formed by a combination of filtration, active secretion and reabsorption. The brain and spinal cord are hollow structures. The two lateral ventricles communicate with the third ventricle through two independent openings called the interventricular foramina. Choroid plexus is a collection of capillaries which arise from some of the arteries supplying the brain. The brain and spinal cord are surrounded by three layers of the connective tissue called meninges. The major source of CSF is the choroid plexus. CSF is a relatively easily accessible fluid which provides the clinician a window to the brain. Hydrocephalus is an increase in the volume of CSF leading to enlargement of cerebral ventricles. The anatomical basis of the blood-brain barriers resides in the special structure of capillaries of the brain. The blood-brain barrier is deficient or weak in some normal and abnormal situations.