For many species, chemical signalling is used to signal the competitive ability of individuals and therefore plays an important role in the breeding system. As breeding generally promotes intrasexual competition, the potential fitness costs associated with competitive behaviour may be mitigated if individuals are able to assess their own ability, and the competitive ability of others, prior to agonistic encounters. Due to their hierarchical social structure and large home-range size, bears (Ursidae) are thought to rely highly on olfactory methods of communication. It is well known that bears rub parts of their body against trees. Selectivity in the trees used and seasonal variation of rubbing behaviour suggest it is a form of scent marking. However, methods of scent deposition and the function of tree marking have received little attention. This study documents tree marking behaviour in brown bears Ursus m'etos to investigate its role as a method of chemical signalling. Using observational research methods, the location of marking trees, frequency of scent marking, time invested in scent marking and receiving scents, and motor patterns exhibited at marking trees, were all investigated for each age sex class in both the breeding and non-breeding season. The observed behaviours were evaluated in terms of their potential fitness benefits to the individuals concerned. Ecological analyses indicated that bears are highly selective in the species, size and location of trees used for marking. Behavioural analyses identified seasonal and inter- sexual variation in tree marking behaviour, as well as a stereotyped pattern of marking. Analysing the time and energy brown bears invest in receiving marks and scent marking indicated variation between age sex classes. Overall, adult males appeared to invest the most time and energy in tree marking, which could indicate that they . gain a net fitness benefit from chemical signalling. The behaviour of other age sex classes at marking trees, particularly subadults and females with young, appears to be dictated by the behaviour of adult males in the area at that time, which causes seasonal variation. Brown bears appear to be highly selective in where they place scent marks, how often they engage in marking behaviour, and how much time and energy they invest in scent marking and investigating scent. It is hypothesised that the function of tree marking is to communicate competitive ability between individuals, with dominant individuals signalling their high competitive ability and receivers detecting theses cues and modifying their behaviour accordingly. Marking trees may also function in individual recognition of conspecifics, and could facilitate scent matching. This study suggests that scent marking plays an important role in the social behaviour of brown bears.