Progressive myoclonus epilepsies (PME) are a group of rare disorders that were initially characterized into three groups on pathological grounds: Lafora disease, lipidoses, and the 'degenerative' forms. 'Degenerative' referred to forms where light microscopy of the brain revealed only neuronal loss and gliosis, without evidence of neuronal storage. With improved pathological, biochemical and clinical analyses, and application of molecular genetics, numerous specific forms of PME have been identified. All are characterized by myoclonus, epilepsy and progressive neurological degeneration. Unverricht-Lundborg disease (ULD) and myoclonic epilepsy with ragged-red fibers (MERRF) are the most common degenerative forms.