Biologists have accumulated abundant evidence to strongly indicate that many species of animals exhibit territorial behavior, that is they demarcate and defend in some way a particular space or territory, especially against conspecifics (members of their own species). The range of species that exhibit such behavior is diverse, including numerous bird species, lizards and salamanders, wolves, deer, a variety of insect species, even fish. Most of the early studies of territorial behavior in animals were of birds, work dating back to the mid-eighteen hundreds.

Territoriality is defined most simply as the attempt by an individual organism or group of organisms to control a specified area, i.e. territory is "any defended area." The area or territory, once controlled, is often bounded by some kind of marker. And, control of territory usually means defense of territory, which may be enforced by the marker, and which may or may not be aggressive. Control of territory means benefits (access to limited resources, reproductive success); defense of territory incurs costs (e.g., stress, time for patrols, etc.).

Habitat selection is closely related to territorial behavior. Territories are usually small segments of larger habitats, and are chosen after assessing and moving in to an appropriate habitat. The spatial arrangement and relations of individuals in a given population are context dependent, and closely related to the dynamics of the population. Spatial relations are coupled to the age, size and sex of individuals, and change and fluctuate according to the appearance of new individuals, the departure or death of others, and the interactions with neighboring individuals, of the same and other species.

Territoriality is found in a wide range of organisms, probably including humans, though the degree to which the human species exhibits territorial behavior remains uncertain and controversial. Social scientists have studied a range of related concepts, including personal space, social space, crowding, privacy, boundary conflicts, and property. Biologists have also contributed to understanding of territorial behavior through the use of related concepts such as spacing and density, home range or domain, micro-ecology, refuging, body zones, and flight distance.