In an integrative approach, this thesis addressed thermal tolerance in temperate and Antarctic fish examining its functions, limits and mechanistic links between the organismic, cellular and molecular level. The role of oxygen in limiting thermal tolerance of the Antarctic eelpout P. brachycephalum was investigated in in vivo NMR experiments. Temperature effects on respiration, blood flow, intracellular pH and tissue oxygenation were studied under normoxia and hyperoxia. Thermal tolerance was limited by the capacities of the circulatory system supplying oxygen to the tissues. At a lower level of organismic complexity, thermal sensitivity of cellular energy allocation was studied in Antarctic fish. Organismic thermal limitations were not reflected at the cellular level. Provided with sufficient oxygen, cellular energy budgets show greater thermal tolerance than the organism. These findings corroborate that capacity limitations of the organismic level constrict thermal tolerance and support the recent concept of a systemic to molecular hierarchy. At the molecular level, temperature sensitive expression of mitochondrial uncoupling proteins (UCP) was studied during acclimation of P. brachycephalum and the temperate eelpout Z. viviparus. Increased levels of UCP may be necessary to regulate high mitochondrial membrane potentials resulting from unchanged capacities in the warm, possibly indicative of an alternative way of mitochondrial warm adaptation in Antarctic fish. The data demonstrate that thermal tolerance of the various levels of organisation in fish differ when studied alone, but in a complex organism are in control of each other, with the highest organisational level showing highest thermal sensitivity. Within a narrow thermal window, slow warm acclimation of the individual appears possible in Antarctic fish, which in an integrated response of all levels of organisational complexity may shift towards a eurythermal mode of life, thus increasing thermal tolerance.