This chapter provides an overview of the various strategies that fishes use to mitigate the effects of temperature change, summarizes how physiologists characterize the thermal niche of fishes and examines the mechanisms fishes use to compensate for the effects of temperature across levels of biological organization. The fundamental thermal niche of a fish species is defined as the range of environmental temperatures over which that species can survive, grow and reproduce. Fishes living in variable environments are typically eurythermal – able to tolerate a wide range of environmental temperatures. The ability of fishes to maintain physiological performance in the face of thermal variation differs among species and is largely reflective of the range of temperatures experienced in their natural environment. For ectothermic fishes, exposure to temperature change causes changes in biochemical processes and metabolism that must be compensated for in order to maintain performance.