The freshwater ostracods living exclusively in interstitial and/or interstitial and cavernous habitats belong to the Candoninae, Pseudolimnocytherinae, Timiriaseviinae, Kliellinae and Darwinulidae. An assessment of the antiquity of several interstitial ostracod groups has been attempted using direct evidence from the phylogenetical affinities between living hypogean and fossil ostracod species, and indirect evidence from the morphological characters of the subterranean ostracods and from the bio- and paleobiogeographical distribution of the different ostracod groups.