Well-hydrated humans can dissipate heat by evaporation far more rapidly than most species and the heat tolerances and evaporative cooling capacities of small endotherms are modest by comparison. Moreover, many birds and small mammals inhabit arid environments with scarce and unpredictable water resources, creating trade-offs between hyperthermia tolerance and dehydration avoidance. The potential for extreme heat events to push small endotherms beyond their physiological tolerance limits is dramatically illustrated by catastrophic mortality events involving birds and bats, the frequency of which is increasing as climate change advances. In this presentation, I review thermoregulation in the heat among birds and small mammals, and link physiology to behavioural and ecological factors that determine sensitivity to very hot conditions. During heat exposure, small endotherms employ facultative, reversible hyperthermia. The maximum body temperatures tolerated vary widely among mammals, from 38-39 °C in marsupials and fruit bats up to ~45 °C in some rodents and small insectivorous bats. Avian maximum body temperatures are typically in the 44-46 °C range, but may be as high as ~ 47 °C. There is increasing evidence from studies of free-ranging populations that small endotherms often maintain very small thermal safety margins, regularly allowing body temperature to approach lethal limits during hot conditions. The maximum air temperatures small endotherms can tolerate during brief heat exposure also vary widely, from 32 °C in temperate-latitude shrews to above 60 °C in arid-zone doves and nightjars. During thermoregulation in the heat, evaporative water losses may occur via several pathways other than sweating. Panting is the primary avenue of heat dissipation for most birds and many mammals, usually accompanied by increases in resting metabolic rate. Among avian taxa in which panting is the primary avenue of evaporative cooling, maximum ratios of evaporative heat loss (EHL) to metabolic heat production (MHP) are typically 2.0 – 2.5. Some taxa, most notably columbid birds, can dissipate substantial heat loads cutaneously, with rates of cutaneous evaporation being determined by microcirculatory adjustments over short time scales and phenotypic flexibility in stratum corneum lipid composition over longer time scales. Among species in which cutaneous evaporation predominates, maximum EHL/MHP values vary between 3.0 and 4.7. Another highly efficient avenue of heat dissipation found in many birds is gular flutter, which can provide the basis for EHL/MHP above 5.0. Many mammals, including marsupials, bats and rodents, spread copious amounts of saliva over their fur to enhance evaporative cooling during heat exposure. A non-evaporative avenue of heat loss that is emerging as important in birds is the beak, with the shunting of blood to the beak vasculature when air temperature is below body temperature providing the basis for rapid heat dissipation in large-beaked species such as toucans and hornbills. Many small endotherms operate close to their physiological limits in hot environments. The increasing temperatures and more frequent heat waves associated with rapid anthropogenic climate change are predicted to cause severe declines among species inhabiting hot regions. These declines will be driven both by acute, lethal effects of extreme heat events and sublethal fitness costs associated with chronic exposure to sustained hot weather. Among desert birds, many of these sublethal fitness costs arise from trade-offs between foraging and thermoregulatory behaviours such as panting and shade-seeking; consequences include progressive loss of body condition, reduced provisioning rates to nests, lower chick growth rates and more frequent breeding failure.