Animals must balance their energy budgets even when confronted with periodic food shortages and/or adverse environmental conditions. Especially, small endothermic animals require large amounts of energy to maintain high and stable body temperatures (Tb) via endogenous heat production. To deal with energetic challenges, many small endotherms are heterothermic, abandon regulation of high Tb and enter a state of torpor resulting in large energy savings. Torpor is used by many bat species because they are small, have high rates of heat loss and rely on fluctuating food resources (e.g. insects, fruit, nectar). Many bats use torpor all year, but the expression of temporal heterothermy can be strongly seasonal especially for temperate and subtropical species, which may hibernate for long periods. Recent advances in our understanding of torpor expression in bats have been made using temperature telemetry for remote data collection of Tb in free-ranging wild individuals from all climate zones. This new knowledge on free-ranging bats has revealed the importance of torpor expression not only for energy conservation but also for other benefits, such as reduction of extrinsic mortality (e.g. predation). On the contrary, dense clustering during hibernation, important for minimizing energy and water loss, may also expose bats to infectious disease. An emerging, cold-tolerant fungal pathogen of bats causes a new disease called white-nose syndrome (WNS), which is devastating populations of multiple species in eastern North America. Given the importance of temporal heterothermy to their biology, and links between torpor expression and mortality from WNS, it is becoming increasingly important to understand the ecology and physiology of torpor in this largely understudied and cryptic mammalian group. Here, we review past and current literature to summarize the importance and evolution of heterothermy in bats.