Odocoileus hemionus species occurs throughout western North America and its southernmost distribution reaches central Mexico, where the historical boundary is not very clear. In this chapter, we present information about its origin and geographic distribution, mainly concerning to mule deer lineage (O. h. crooki, eremicus, fuliginatus, peninsulae, cerrosensis and sheldoni) that are distributed in the arid zones of Mexico. Many biological and ecological aspects of mule deer have been widely studied in the southwestern USA, but in Mexico few studies have been done in arid lands (e.g. its diet, population, genetics), so we present a compilation of the information that is currently available for the species in these regions. A synthesis about the characteristics of the three most important habitat requirements (food, water and cover) that can become limiting factors for mule deer populations in arid zones is presented here; these habitat components have only been estimated in some occasions in Mexico. The quality and availability of these requirements are highly affected by overgrazing, and it has put the survival of many deer populations at risk. The mule deer is classified as one of the species of least concern in terms of conservation, although its populations and distribution areas have been reduced, even with some local extinctions. Mule deer provide an excellent example of the need to evaluate species at the regional and national levels to arrive at a more realistic classification of their conservation status.