The authors study the tumors derived from inclusions of foetal ectodermal tissues, either of Malpighian epithelium (epidermoid cysts) or those also including dermal elements (dermoid cysts). The authors criticize the words cholesteatoma and pearly tumor, often applied to the former growths. The rareness of these cysts among nervous system tumors (particularly spinal epidermoids and diploic dermoids) is stressed. Sluggish development and incidence in youth are clinical features of both. The neurological symptomatology is extremely varied, since these tumors can be found in ?very site of the neuraxis. Some forms show peculiar signs which can point to their etiologic diagnosis. The authors report 12 cases of epidermoid (including 3 of the spinal canal) and 2 cases of dermoid cysts (including one of the diploe). Attention is called to eventual changes of the cerebrospinal fluid that may lead to erroneous diagnosis such as neurosyphilis and cysticercosis of the nervous system. The general rules and results of the surgical management are briefly analysed. In their own cases the authors had three post-operative deaths and three deaths not related to craniotomy (two preoperative and one due to an intervening cause: perforation of gastric ulcer); in the other cases improvement was achieved by surgery.