This chapter focuses on three factors that pervasively affect the metabolic rates of fishes: body mass, temperature, and hypoxia. Metabolism is the sum of the chemical reactions within the body that convert food to energy molecules and building blocks for growth, and which use these compounds to sustain the cellular energy demands that support life. Ectotherms, including almost all fishes, are particularly temperature sensitive, because their body temperature matches the surrounding environment. Metabolic rates of fishes vary widely and in association with both lifestyle and trophic level. All living things, including fishes, must harness energy and materials from their environment in order to maintain homeostasis, grow, and reproduce. In fishes and other ectotherms under normal environmental conditions, this is referred to as the standard metabolic rate, which is the energy expenditure of a post-absorptive, non-reproducing, and quiescent animal at a defined temperature. Metabolic rate is influenced by numerous physiological and environmental factors that affect energy supply and/or demand.