Huntington's disease is a progressive and fatal neurological disorder caused by the expansion of a CAG trinucleotide repeat in exon 1 of the gene coding for a protein of unknown function that has been named huntingtin. The exact cause of neuronal death in Huntington's disease is unknown; however, the leading hypothesis is that of excitotoxicity and apoptosis induced by a defect in energy metabolism that may be caused by oxidative stress. How mutant huntingtin might cause these processes is unknown. New animal and cell models provide insights into the mechanism of pathogenesis and the search for the development of effective therapies.