Mammalogy is the branch of zoology that deals with animals belonging to the class Mammalia. Two of the characteristics that distinguish mammals from the other vertebrates are the presence of hair on at least part of their bodies and the fact that females nourish their young with milk secreted by their mammary glands. About 5,413 species of living mammals, divided into 30 orders and 149 families, are currently recognized. The larger mammals, such as carnivores and ungulates, are among the most familiar and charismatic of animals, although the majority of mammal species are small rodents or bats that go largely unnoticed by most people. Mammals possess morphological, physiological, and behavioral adaptations for an amazing variety of habitats and lifestyles. Some mammals spend most of their time underground or in trees, some are completely aquatic, some live in extremely arid deserts, and some can even fly. Mammalogists tend to specialize in the study of the systematics, anatomy, physiology, ecology, or behavior of a particular taxonomic group or assemblage of mammals, such as bats, canids, or desert rodents.

The Illinois Natural History Survey Mammal Collection contains 15,418 catalogued lots (skins and skulls combined) of 157 species from 25 families. These specimens are primarily used as comparative material to identify specimens brought to the Survey. Just over half (51%) of the specimens are from Illinois and many were collected between 1930 and 1950 in Illinois and surrounding states.