ABSTRACT Rates of force development, contraction and relaxation of vertebrate skeletal muscle are temperature dependent with Q10 values of approximately 2. Maximal forces developed have a low or negative thermal dependence. The functional basis of these patterns is poorly understood. Muscle performance generally does not acclimate. There appears to have been some evolutionary adaptation among species and classes to different thermal regimes, such that muscles from cold-adapted species maintain better mechanical performance at low temperatures than do those from warm-adapted animals. However, rate processes remain strongly thermally dependent even in animals with low or variable body temperatures. This thermal dependence of muscle in vitro is reflected in behavioural performance: maximal force generation in vivo is temperature independent and time-dependent activities are more rapid at higher muscle temperatures.