Intraspecific variation in key traits such as tolerance of warming can have profound effects on ecological and evolutionary processes, notably responses to climate change. We review the empirical evidence for three primary elements of intraspecific variation in tolerance of warming in fishes. The first is purely mechanistic, that tolerance varies across life stages and as fishes grow to maturity. The limited evidence indicates strongly that this is the case, possibly due to universal physiological principles. The second is intraspecific variation that is due to phenotypic plasticity, also a mechanistic phenomenon that buffers individuals’ sensitivity to negative impacts of global warming in their lifetime, or to some extent through epigenetic effects over successive generations. Although the evidence for plasticity in tolerance to warming is extensive, more work is required to understand underlying mechanisms and to reveal whether there are general patterns. The third element is intraspecific variation based upon heritable genetic differences in tolerance, which underlies local adaptation and may define long term adaptability of a species in the face of ongoing global change. There is clear evidence of local adaptation and some evidence of heritability of tolerance to warming, but the knowledge base is limited with detailed information for only a few model or emblematic species. There is also strong evidence of structured variation in tolerance of warming within species, which may have ecological and evolutionary significance irrespective of whether it reflects plasticity or adaptation. Although the overwhelming consensus is that having broader intraspecific variation in tolerance should reduce species vulnerability to impacts of global warming, there is not sufficient data on fishes to provide insight into particular mechanisms by which this may occur.