Arachnoid cysts are space-occupying lesions filled with CSF-like content and surrounded by a membrane resembling arachnoid matter. They are regarded as a development abnormality of the arachnoid, originating from a splitting or duplication of this membrane. However, precise etiology and natural history remain controversial. Different hypotheses have been developed including agenesis of brain structures, arachnoiditis, active fluid secretion, and pulsatile pump. We present a review of the literature concerning these items and report one case of a suprasellar arachnoid cyst in which a slit-valve mechanism was observed by means of cine-mode MRI preoperatively and confirmed during the endoscopic intervention.