These are congenital lesions which arise during the development of the nervous system, from a split in the membrane surrounding and investing the brain.  They are located most commonly in the middle fossa (near the temporal lobe), although they may be located in other portions of the brain as well.  They may present with headaches, seizures or weakness on one side of the body.  These cysts are often watched, but may be operated upon if they become symptomatic.

In the CT of the head seen here, an arachnoid cyst is seen sitting in front of the left temporal lobe. The cyst is also seen in the operating room.