This presentation will be focused on fish species living in the polar regions. The two regions differ in both the tectonic history and the fish fauna composition. The fish fauna around the Antarctic continent was isolated by the opening of Drake Passage and is today represented by 313 species representing only around 1% of the known fish species. The composition of the Antarctic fish fauna today is unique and dominated by endemic perciform notothenioids with a monophyletic background. In contrast, the dominant fish fauna of the Arctic are polyphyletic and include functionally diverse gadids, cottids, salmonids, pleuronectids, and chondrichthyans which have representatives with wide latitudinal distribution. Both polar regions shows extremely low water temperature during the winter months but in the Arctic a larger variation in temperature is seen during the summer with temperature reaching up to 9°C. In contrast, many areas around the Antarctic continent have stable water temperatures below 0°C throughout the year. A worrying aspects of especially the Arctic regions is the very rapid change in climate due to the Arctic amplification, it is estimated that the speed of change is 3–4 times faster compared to the global average. Another linked problem is the decrease in pH of the water (ocean acidification) due to increase in carbon dioxide levels. This leads to a combined stress (temperature and pH) that might affect the animals more than the either pH or temperature alone. The adaptations to the extreme environment and possible consequences of changes in this environment of both Antarctic and Arctic fish species will be discussed both in terms of consequences on the individual leveled but also on a higher ecological level.