South America harbours the most diverse freshwater fish fauna in the world, and recent estimates point to between 6000 and 8000 species in the Neotropical region. Such fauna has diverse historical origins, either having invaded and diversified in fresh water from marine ancestors during the Palaeogene or being isolated on the continent since the end of the break-up of Gondwana in the Cretaceous. Taxonomic, morphological and ecological diversity of South American freshwater fishes is dramatic, as are the myriad freshwater habitats they inhabit. Unfortunately, many of these habitats are severely threatened by deforestation, water divergence for irrigation, industry and other uses by humans, hydroelectric damming, mining, pollution and invasive species. Despite these multiple threats, there are very few on-the-ground conservation initiatives in South America, although assessments of species-extinction risks have been produced at regional and subregional levels in different countries.