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Defined as fungi that grow better at 25 °C than at 45 °C, thermophilic fungi were discovered more than a century ago. Nevertheless, little is known about the natural roles and distribution of these organisms. Although common in “sun-heated soils” and other natural substrates they have most often been recovered from manmade composts, and one hypothesis suggests that they evolved as decomposers in natural compost. This hypothesis suggests that propagules found outside compost have been dispersed by wind, an idea that seems nearly impossible to reconcile with their high frequency and broad distribution. In this chapter we briefly review the biology, history, and evolution of thermophilic fungi. We also present new results from ongoing efforts to map the range of habitats from which thermophilic fungi can be obtained. We have isolated thermophilic fungi over small and large spatial scales. Our surveys have focused on soil, litter, and herbivore droppings sampled from diverse ecosystems (deserts, grasslands, and forests) across eight western states, Mexico and Canada—from southern deserts to alpine ecosystems in Colorado and Montana. Our results show that thermophiles can be isolated readily from all of these substrates at nearly every latitude and elevation. We observed that the success of recovering thermophilic fungi from soil decreases with increasing latitude. During this survey, we also discovered that several species of thermophilic fungi can survive storage in soil samples for several years at −80 °C.