This chapter reviews the major advances in autosomal recessive and autosomal dominant ataxias, discusses the use of genetic tests in these disorders, and summarizes some current ideas regarding pathogenesis. It also presents a list of the autosomal recessive ataxias that have been genotypically characterized to date. Mutations in ataxia with isolated vitamin E deficiency (AVED) are scattered throughout the gene and some of them may be associated with a mild phenotype, late onset, retinitis pigmentosa, and retained reflexes. A syndrome of ataxia associated with optic atrophy, visual loss, and cochlear degeneration has been mapped to chromosome. The spinocerebellar ataxia (SCAs) exhibits many phenotypic similarities so that it is almost impossible to diagnose the genotype from the phenotype alone. Many persons from families with ataxia will request predictive testing and occasionally prenatal testing. Disease-modifying therapies are under investigation and include antioxidants and drugs that may modify excitotoxicity or apoptosis.