Posterior fossa cysts are frequently identified on MR studies. This paper takes a different approach to analyzing these cysts based on the pathology of the cyst wall and the embryology of the hindbrain, choroid plexus, and meninges. The type of cyst depends on the histologic components of the cyst wall. Frequent types of posterior fossa cysts are arachnoid, Blakes pouch, and cysts associated with Dandy Walker malformation. All of these cysts may mimic the others in terms of position of the torcula, vermian abnormalities, and mass effect on the cerebellum and occipital bone. A clue to the nature of the cyst may be the position of the choroid plexus in the fourth ventricle; normal in arachnoid cyst, absent in Dandy Walker malformation, and displaced into the superior cyst wall in Blakes pouch. When the cyst wall histology is not known, it is suggested to use a descriptive term such as retrocerebellar cyst.