Yak (Bos grunniens) are members of the Artiodactyla, family Bovidae, genus Bos. Wild yak are first observed at Pleistocene levels of the fossil record. We believed that they, together with the closely related species of Bos taurus, B. indicus and Bison bison, resulted from a rapid radiation of the genus towards the end of the Miocene. Today domestic yak live a fragile existence in a harsh environment. Their fitness for this environment is vital to their survival and to the millions of pastoralists who depend upon them. Their wild counterparts are further marginalised to the most inhospitable areas of the Qinghai-Tibetan Plateau. Recent introduction of cattle (mainly B. taurus), in an attempt to improve stock, may threaten the integrity of the yak genetic make-up with the consequence of reducing fitness with respect to cold temperature and high altitude survival. Mitochondrial DNA (mtDNA) is ideally suited as a tool for studying population genetics because it has the unique features of maternal inheritance, a relatively fast rate of evolution and lack of recombination. Population history is traced back through the maternal line, which excludes the male genome and thus establishes a simple underlying pattern. It is possible to investigate the recent history of domestication due to the fast evolutionary rate of mtDNA. Lack of recombination between the mtDNA of yak and cattle is informative for the study of introgression. This paper discusses the genetic diversity of yak expressed in terms of sequence variations found in the displacement loop (D-loop) of mtDNA. A comparison has been made between these values and those found within other domestic species, primarily taurine cattle. An exact time for the domestication of yak is difficult to discern from archaeological data; here we estimate a time based upon a molecular clock of approximately 5000 years ago. Finally we shall feature the mtDNA introgression observed in yak populations.