The hereditary ataxias are a highly heterogeneous group of disorders phenotypically characterized by gait ataxia, incoordination of eye movements, speech, and hand movements, and usually associated with atrophy of the cerebellum. There are more than 35 autosomal dominant types frequently termed spinocerebellar ataxia and typically having adult onset. The most common subtypes are spinocerebellar ataxia 1, 2, 3, 6, and 7, all of which are nucleotide repeat expansion disorders. Autosomal recessive ataxias usually have onset in childhood; the most common subtypes are -Friedreich, ataxia-telangiectasia, ataxia with oculomotor apraxia type 1, and ataxia with oculomotor apraxia type 2. Four autosomal recessive types have dietary or biochemical treatment modalities (ataxia with vitamin E deficiency, cerebrotendinous xanthomatosis, Refsum, and coenzyme Q10 deficiency), whereas there are no specific treatments for other ataxias. Diagnostic genetic testing is complicated because of the large number of relatively uncommon subtypes with extensive phenotypic overlap. However, the best testing strategy is based on assessing relative frequencies, ethnic predilections, and recognition of associated phenotypic features such as seizures, visual loss, or associated movement abnormalities.