Beavers are highly territorial and actively defend a territory that includes nutritional resources, one or more rest sites, a family overwintering site, and a reproduction site. New territories are normally established during early autumn when dispersing subadults settle down in a new area of their own, but this can vary considerably. Each family group scent marks its territories with castoreum and/or anal gland secretion. Territory size is generally measured as the length of shoreline but can also be expressed as a combination of terrestrial and aquatic patches used by the beavers in a two-dimensional space. The territorial size depends on many factors such as habitat quality and quantity, habitat type (stream, river, or pond), density of beavers, social factors (sex and age), time of year (season), and settlement pattern. Beavers patrol their territory to look out for intruders and potential predators and scent mark their territory with fluid from their castor sacs and/or secretions from their anal glands. Territorial defence can include direct fighting or even more unusual warning behaviours such as boundary stick displays. The duration of territory occupancy and its importance for the fitness of beavers are also discussed. Further, this chapter explores beaver communication using odours, sounds, tail slapping, poses, and other movements. Beavers predominantly communicate via odours, but the importance of all these modalities is explored. Lastly, population estimations, growth, fluctuations, and densities of beaver populations are discussed.