Burrowing behavior is widespread among mammals and has generated a diverse array of adaptive responses to the physical demands of this lifestyle. While extensive research has been devoted to the morphological, ecological, and evolutionary implications of burrowing, it remains difficult to compare burrowing adaptations between mammals of widely divergent ancestry. A reliable quantitative proxy for fossoriality (burrowing) is necessary for such comparisons as well as for detailed descriptions of ecology from specimens of rare, extinct, and fossil mammals. This study presents several quantitative indices of the morphology of burrowing mammals based on 20 measurements of skull and skeletal morphology taken from 123 different mammalian species, both burrowing and nonburrowing. Discriminant analyses revealed that these quantitative characters successfully distinguish nonburrowing taxa from those that are adapted to a burrowing lifestyle. Additionally, more subtle distinctions between subterranean taxa (which rarely emerge above ground) and other burrowers as well as between mammals using different methods of burrow excavation were identified from these characters. A test of these indices using 6 extinct species yielded results consistent with more-detailed descriptions of the functional morphology of these taxa, indicating that our quantitative proxies provide an important basis for comparisons of fossorial adaptations across divergent mammalian clades.