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Many birds reduce rest-phase energy demands through heterothermy, physiological responses involving facultative, reversible reductions in metabolic rate and body temperature (Tb). Here, we review the phylogenetic distribution and ecological contexts of avian heterothermy. Heterothermy has been reported in 140 species representing 15 orders and 39 families. Recent work supports the view that deep heterothermy is most pronounced in phylogenetically older taxa whereas heterothermy in passerines and other recently diverged taxa is shallower and confined to minimum Tb > 20°C. The reasons why deep heterothermy is absent in passerines remain unclear; we speculate an evolutionary trade-off may exist between the capacity to achieve low heterothermic Tb and the tolerance of hyperthermic Tb. Inter- and intraspecific variation in heterothermy is correlated with factors including foraging ecology (e.g., territoriality and defense of food resources among hummingbirds), food availability and foraging opportunities (e.g., lunar phase predicts torpor use in caprimulgids), and predation risk. Heterothermy also plays a major role before and during migration. Emerging questions include the magnitude of energy savings associated with heterothermy among free-ranging birds, the role phylogenetic variation in the capacity for heterothermy has played in evolutionary radiations into extreme habitats, and how the capacity for heterothermy affects avian vulnerability to rapid anthropogenic climate change., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology.)