The progressive myoclonus epilepsies (PMEs) comprise a group of rare and heterogeneous disorders defined by the combination of action myoclonus, epileptic seizures, and progressive neurologic deterioration. Neurologic deterioration may include progressive cognitive decline, ataxia, neuropathy, and myopathy. The gene defects for the most common forms of PME (Unverricht-Lundborg disease, Lafora disease, several forms of neuronal ceroid lipofuscinoses, myoclonus epilepsy with ragged-red fibers [MERRF], and type 1 and 2 sialidoses) have been identified. The prognosis of a PME depends on the specific disease. Lafora disease, the neuronal ceroid lipofuscinoses, and the neuronopathic form of Gaucher disease have an invariably fatal course. In contrast, Unverricht-Lundborg disease has a much slower progression, and with adequate care many patients have a normal life span. The specific diseases that cause PME are diagnosed by recognition of their age of onset, the associated clinical symptoms, the clinical course, the pattern of inheritance, and by special investigations such as enzyme measurement, skin/muscle biopsy, or gene testing.