Arachnoid cysts are cerebrospinal fluid covered by arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the arachnoid membrane, one of the three membranes that cover the brain and the spinal cord. Arachnoid cysts are a congenital disorder, and most cases begin during infancy; however, onset may be delayed until adolescence.

Meningiomas are the second most common primary tumor of the central nervous system, arising from the arachnoid "cap" cells of the arachnoid villi in the meninges. These tumors are usually benign in nature; however, they can be malignant.