The prevalence and persistence of Huntington’s disease (HD) is crucially informed by the causative mutation. Diagnostic and predictive testing has enabled a new era of epidemiologic study of HD, whereby only those who carry an expanded CAG repeat are included in such measures. In Western populations, estimated prevalence of the disease is higher following the introduction of genetic testing, and prevalence may also be increasing in absolute terms. There are worldwide differences in the prevalence of HD by ethnicity and population, which may be accounted for in part by genetic diversity of the CAG repeat and the surrounding haplotype. HD is endemic to all populations, but is most common in populations of European ancestry in which specific disease haplotypes are found. New mutations maintain HD in a population, and genetic differences by population may contribute to differences in the de novo mutation rate.