Many hundreds of species of wildlife worldwide are dependent on tree hollows (cavities) for their survival. I reviewed the published literature for hollow-using Australian birds and microbats to document their tree-hollow requirements and to guide future research and management. Such information is vital to the conservation of these species. The hollow requirements of only 35 of 114 hollow-using bird species and 15 of 42 hollow-using microbat species were documented in some detail. This overall paucity of information limits the ability to manage for the future requirements of species. However, some generalisations can guide management until further studies are conducted. Most species used a variety of available tree species, and the extensive use of dead trees probably reflects the high likelihood of these trees containing hollows. Birds (other than large parrots) and bats chose hollow entrances of a size close to body width. Large parrots require large hollows, with a preference for large vertical spouts and trunk hollows. Few birds or bats demonstrated an absolute requirement for high (>10 m) tree hollows, with most (70%) using some hollows with entrances ≤5 m above ground. Temperature has been postulated to influence roost selection among microbats because it enables passive rewarming from torpor and there is some evidence from Australian bats to support this. Many studies suggest a future shortage of hollow-bearing trees. Currently, artificial hollows appear to be the most likely interim solution to address this. Knowledge of the natural hollow requirements of species can be used to refine artificial-hollow designs. An increase in research effort is needed to address the many gaps in knowledge that currently exist. Priorities for research include (1) many additional studies to document the characteristics of the hollow-bearing trees used by species of microbat, (2) the need to conduct long-term bioregional studies of hollow-bearing tree attrition to help identify where management responses are most needed and (3) investigating whether fire plays a significant role in the creation of tree hollows of a range of size classes and therefore may have a management use. Such information has broad relevance because it will provide ecological insight that can be applied to the management of hollow-using birds and bats elsewhere in the world.