The Division of Mammals contains over 327,000 catalogued specimens and is among the 3 largest in the world. Specimens date back to the 1880's with the majority documenting the rapid environmental change that has occurred since the 1950's. The collections are taxonomically broad, representing 25 orders, 106 families, 543 genera, ~1,750 species. The majority from the Orders Rodentia (249,000), Chiroptera (27,000), Carnivora (20,000), Eulipotyphla (16,000) and Artiodactyla (7,800). The collections are world-wide in scope (78 countries and all 50 US states) with particularly strong holdings from Western North America (225,000 specimens), Beringia and high latitudes (39,000 from Alaska, Russia and Canada), Mongolia (6,500 specimens and parasites), and Latin America (10,200 specimens from Bolivia, 10,000 from Panama, 7,000 from Chile, and 5,400 from Argentina). The collection contains 89 holotypes or paratypes, 185 parasite symbiotypes and 22 virus symbiotypes. Important collections integrated into the MSB include the USGS Biological Surveys collection (30,000), the UIMNH (Hoffmeister) Collection (33,000), and the Rausch Collection (4,000 specimens and parasites). Specimens range from traditional skin/skull and fluid vouchers to "holistic vouchers" containing skin, skull, post-cranial skeleton, up to seven tissue types (heart, kidney, liver, lung, spleen, muscle, blood), cell suspensions, and ecto and endo parasites. Additionally, frozen tissue samples are available for about 200,000 individual mammals and date back to the late 1970's. 37,000 specimens have serology data associated with hantavirus surveillance programs in the Americas. The MSB houses an extensive archive of field journals and catalogues that date to the 1900's and are associated with specimens held in the collection. The collections are growing rapidly through the active research programs of curators, staff and students and ongoing collaborations with other institutions and governmental agencies.