Fish live in an environment where temperature routinely changes. These temperature fluctuations place demands on the circulatory system, particularly the heart, which must pump sufficient blood to meet temperature-related changes in tissue oxygen demand. The heart of fishes clearly has temperature limits within which its function is constrained. However, these limits differ among species, and depend on the time allowed to adjust to the temperature change. These adjustments can be phenotypic or phylogenetic, and involve changes in cardiac morphology, its nervous control, or the cellular processes mediating contraction frequency and force development.