Postojnska jama is a part of the Postojna-Planina cave system (PPCS), the most biologically diverse cave in the world. Postojnska jama is a locus typicus of at least 37 animals, among them also cave beetle Leptodirus hochenwarti, the first recognized cave adapted animal. PPCS consisting of 17 and 6 km of passages, respectively, connected by 2 km of flooded corridors, the PPCS has more known species of stygobionts than any other cave or other subterranean site. The sinking river in the main passages is inhabited by a rich assortment of stygobionts, stygophiles, and accidental surface species. Both streams drain a catchment area of over 800 km2. Above the cave passages is the rich epikarst community. Among the 48 stygobionts is the European cave salamander Proteus anguinus anguinus. Both the snail (8 species) and crustacean (16 species) faunas are rich. The marine origin of some of the stygobiotic species is evident in the hydrozoan Velkovrhia enigmatica and the cirolanid isopod Monolistra racovitzae racovitzae. Three species of the amphipod genus Niphargus as well as populations of Asellus aquaticus were isolated in the cave at different times. The Postojna-Planina Cave is one of the best-studied caves in the world, and parts of it have been heavily visited by tourists since 1818. There are currently installed underground meteorological stations, and sites are defined for permanent water and biodiversity monitoring with the respect to human impact.