Arachnoid cysts are benign congenital cystic brain lesions that are due to abnormal arachnoid division during development. The wall of the cyst is mostly arachnoid, glial and soft meninges, and the cyst contains cerebrospinal fluid-like cyst fluid. The cysts are located on the surface of the brain, in the cerebral fissure and the brain pool, and do not involve the brain parenchyma. Most of them are solitary and a few are multiple. The disease is mostly asymptomatic, but large ones may compress both brain tissue and skull, which may produce neurological symptoms and craniofacial developmental changes. It is more common in children and adolescents, more common in males, and more common on the left side than on the right side. Arachnoid cysts can be classified as congenital or secondary (traumatic or post-infectious) depending on the cause. Congenital arachnoid cysts are pockets of cerebrospinal fluid enclosed within the arachnoid membrane and do not communicate with the subarachnoid space. Secondary ones form cysts in the subarachnoid space due to adhesions in the arachnoid membrane and contain cerebrospinal fluid.