A tree hollow or tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree. They are found mainly in old trees, whether living or not. Hollows form in many species of trees, and are a prominent feature of natural forests and woodlands, and act as a resource or habitat for a number of vertebrate and invertebrate animals.

Hollows may form as the result of physiological stress from natural forces causing the excavating and exposure of the heartwood. Forces may include wind, fire, heat, lightning, rain, attack from insects (such as ants or beetles), bacteria, or fungi. Also, trees may self-prune, dropping lower branches as they reach maturity, exposing the area where the branch was attached. Many animals further develop the hollows using instruments such as their beak, teeth or claws.

The size of hollows may depend on the age of the tree. For example, eucalypts develop hollows at all ages, but only from when the trees are 120 years old do they form hollows suitable for vertebrates, and it may take 220 years for hollows suitable for larger species to form.

Hollows in fallen timber are also very important for animals such as echidnas, numbats, chuditch and many reptiles. In streams, hollow logs may be important to aquatic animals for shelter and egg attachment.

Hollows are an important habitat for many wildlife species, especially where the use of hollows is obligate, as this means no other resource would be a feasible substitute. Animals may use hollows as diurnal or nocturnal shelter sites, as well as for rearing young, feeding, thermoregulation, and to facilitate ranging behaviour and dispersal. While use may also be opportunistic, rather than obligate, it may be difficult to determine the nature of a species' relationship to hollows—it may vary across a species' range, or depend on climatic conditions.

Animals will select a hollow based on factors including entrance size and shape, depth, and degree of insulation. Such factors greatly affect the frequency and seasonality of hollow use.

Especially in Europe, entomologists are interested in the use of hollows by invertebrates. One beetle associated with hollow trees, Osmoderma eremita, has been given the highest priority according to the European Union's Habitat Directive.