Animals expend energy to process information, to forage for and digest food, to move, to grow and reproduce, and to generate or dissipate heat. Measures of metabolic rate integrate these processes, so it is reasonable to expect that variation in metabolic rate should be related to variation in fitness, but empirical demonstrations of such associations are surprisingly rare. Among- and within-species variation in body size and temperature are recognized as primary determinants of metabolic rate, but considerable variation remains once these factors are accounted for. Understanding the causes and consequences of this variation in metabolic rate is a key problem facing the field of evolutionary physiology. Using data drawn from phenotypic, quantitative genetic, and phylogenetic comparative studies, I will discuss the association between mass-independent variation in metabolic rate and fitness for a range of animals including lizards, cockroaches, and mammals. I will then draw upon quantitative genetic and experimental evolution studies of model and non-model species to provide a background to the microevolutionary processes that shape among-species variation in metabolic rate, and compare the patterns observed at the microevolutionary scale with those observed at the macroevolutionary scale.