The Denver Museum of Nature & Science Mammal Collection currently consists of approximately 21,000 specimens, which includes 20,000+ cataloged specimens. The collection spans 1870 to the present, is worldwide in coverage, and includes specimens from the three major extant mammalian lineages (monotremes, marsupials, and placentals) distributed across 21 orders, 266 genera, and 381 species. The collection’s primary strength is its focus on the southern Rocky Mountains and Great Plains. Approximately 82% of the collection is from the western United States, with specimens from Colorado (73%) representing the largest percentage. Small mammals, such as shrews, rodents, lagomorphs, and bats, constitute the majority (84%) of specimens in the collection. The composition of the collection is primarily study skins and skeletal material, with a growing wet collection. High-quality specimen data, frozen tissues, and parasites are associated with most of the specimens archived since 2006.