A growing number of neurodegenerative diseases have been found to result from the expansion of an unstable trinucleotide repeat. Over the past 6 years, researchers have focused on identifying the mechanism by which the expanded polyglutamine tract renders a protein toxic to a subset of vulnerable neurons. In this review, we summarize the clinicopathologic features of these disorders (spinobulbar muscular atrophy, Huntington disease, and the spinocerebellar ataxias, including dentatorubropallidoluysian atrophy), describe the genes involved and what is known about their products, and discuss the model systems that have lent insight into pathogenesis. The review concludes with a model for pathogenesis that illuminates the unifying features of these polyglutamine disorders. This model may prove relevant to other neurodegenerative disorders as well.