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Most reptiles that lay eggs bury them in soil or, in the case of certain crocodilians, in mounds of decaying vegetation. The gaseous environment that these buried eggs experience can be very different from those of the atmosphere (Seymour & Ackerman, 1980). By contrast, avian eggs are usually exposed to more or less atmospheric gaseous conditions (Walsberg, 1980). An exception is the Megapodiidae, a family of galliform birds confined to the Australasian region, which bury their eggs in soil or mounds of decaying vegetation in a manner similar to that of reptiles (Frith, 1956 «). In this chapter we discuss the similarities and differences in incubation biology of buried eggs of reptiles and megapodes and compare their gaseous and thermal environment to that of open nesting birds. All avian eggs have hard calcareous shells, so our comparisons tend to focus on rigid-shelled reptilian eggs. Most of the information for megapode eggs comes from just two species, the malleefowl (Leipoa ocellata) and Australian brush turkey (Alectura lathami). Similarly, information on rigid-shelled reptilian eggs relies heavily on the crocodilians Crocodylus johnsoni, Crocodylus porosus, and Alligator mississippiensis.