This book encompasses basic and clinical reports on the cerebellum and its primary atrophic disorders, the cerebellar degenerations. Rapid progress has been made in undestanding the organization and function of the cerebellum at the neuronal, synaptic, and molecular level. Of particular importance has been the identification of the chemical transmitters utilized by the cer ebellar cellular systems. More than any other brain region, the cerebellum utilizes amino acids as its main excitatory and inhibitory neurotransmitters. Excitatory amino acid transmitters, in addition to serving neuronal com munication, may also mediate trophic and toxic effects, and as such, they may playa role in neurodegenerative processes. The cerebellar degenerations were among the first human disorders with primary system atrophy to be studied clinically and pathologically. This field of clinical cerebellar sciences, no longer confined to the previously known descriptive level, is now advancing rapidly, propelled by rapid advances in neuroimaging, immunology, and molecular biology. The advent of CT, MRI, and PET has in recent years permitted the study of central nervous system alterations in living patients, thus contributing substantially to the accuracy of the diagnosis and the classification of these disorders. The nosology of cerebellar degenerations, which has been the subject of much debate for over a century, is presently a dynamic field, with new entities being recognized and old "classic ataxias" being redefined in the light of new genetic evidence.