Publisher Summary This chapter describes polar fishes from a biogeographical point of view, employing areas with major shifts in species composition as borders for the Arctic and Antarctic areas. These are often defined by oceanic frontal systems and topographical structures, such as submerged ridges. The two polar areas of the world contain 538 species of fish, 289 in the Arctic and 252 in Antarctica. The Arctic fish fauna displays a number of interesting adaptations to the harsh environment that is also reflected in the distribution patterns. The Arctic fish fauna is dominated by phylogenetically young families, with few representatives of old families, particularly in deep waters. In the Antarctic, ice cover plays a more important role in fish distributions than salinity. Deepwater sampling is inadequate in Antarctica and most deep‐sea species are known from very few specimens.