Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, cognitive, and psychiatric symptoms. An unstable CAG trinucleotide repeat expansion within the first exon of the responsible gene "IT15", encoding huntingtin, was identified. The clinical phenotype strongly correlates with the number of CAG repeat. The mutant huntingtin is expressed ubiquitously as same as the wild-type huntingtin, suggesting a toxic gain of function of the mutant huntingtin. The identification of huntingtin-interacting proteins and intranuclear aggregates containing a fragment of mutant huntingtin provide new insights into the pathophysiological mechanisms underlying HD. Moreover, the progress in transgenic animal models for HD will be critical for understanding the development of HD and for the testing of new therapeutics.