The Huntington's disease (HD) locus was one of the first disease associated loci to be mapped using restriction fragment length polymorphisms. The prevalence of HD in populations of Western European descent is usually given as approximately 4-10 per 100,000. HD onset has been described as between the ages of 2 and 87 years. HD can produce a wide range of phenotypic presentations and, as the disease progresses, the signs and symptoms often change. The most well recognized neuropathological classification is the Vonsattel grade. The HD predictive testing guidelines recommend that individuals at risk are seen for two to four counseling sessions spread over a three-month period, before disclosure of the test results. Diagnostic testing for HD usually is undertaken by neurologists when patients present with the neurological signs and symptoms of the disease. Currently, there are several large clinical trials ongoing, including high-dose creatine, coenzyme Q10, and memantine.