The heart must provide its own energy requirements to sustain its continuous contractile performance and other physiological functions. This chapter provides an integrated overview of cardiac energy metabolism in fish with a particular emphasis on: maintenance of cardiac energy state; biochemical strategies for energy production; and energetic requirements to maintain contractile function, ion pumping across membranes, and protein synthesis. Major advances in our understanding of fish cardiac metabolism have come from studies of cellular ultrastructure, ion regulation, enzyme activities, select proteins involved in energy metabolism, fuel selection and energy reserves, intracellular metabolites, cardiac adaptability, and physiological performance of cardiac preparations and intact animals. Total energy expenditure of the contracting heart varies between species and includes basal and active components. The importance of myosin-bound ATP phosphatase (ATPase) for contractility, Na+/K+-ATPase for cellular ion homeostasis, and Ca2 +-ATPases for myocardial relaxation are highlighted. Fish hearts rely on well-established metabolic pathways to regenerate ATP, however, species differences are apparent. Under normoxic conditions, mitochondria produce most of the ATP used by the fish heart using aerobic metabolism and a variety of energy substrates. However, during O2-limiting conditions, anaerobic metabolism (glycolysis) becomes the major source of ATP production, despite an inherently limited capacity compared with oxidative phosphorylation. Cold temperature can also compromise several cellular processes related to cardiac energy metabolism, and yet, some fish demonstrate positive compensation of enzyme activities following cold acclimation and acclimatization. Overall, there are numerous, inter-related factors that underlie cardiac energy production and utilization.