Hereditary ataxias encompass a series of syndromes basically characterised by progressive cerebellar ataxia of slow clinical course (occasionally, periodic ataxia or spastic paraparesis) and primary spinocerebellar degeneration. The prevalence ratio of these syndromes in Spain is 20 cases per 100,000 inhabitants. Initially the ataxias were classified on the basis of clinicopathological criteria. Starting from the seminal papers by Harding published 20 years ago, a clinicogenetic classification was introduced that has given way to the present molecular classification. There have been localised about forty loci. In dominant ataxias the most frequent molecular defect is a dynamic CAG expansion responsible for abnormal polyglutamine tract transcription. The identification of such molecular defect has made it possible detection of gene carriers in clinical practice, this involving both presymptomatic and prenatal diagnosis; moreover, such molecular discoveries have contributed to develop a new pathogenetic era. A homozygous and intronic GAA expansion is the molecular basis of Friedreich's ataxia. This finding has also made it possible a molecular diagnosis in clinical practice. Molecular studies have demonstrated that hereditary spastic paraplegia is another heterogeneous genetic disorder.