Behavioural modifications have less interested neuroscience than cognition these last decades, but have nevertheless a tremendous impact on outcome of neurological diseases. Moreover, they have less been taken into account in rehabilitation than cognitive deficits, while they are probably the most important value to analyse when a successful familiar and social reintegration is being discussed. In the clinical situation behaviour refers to the manner in which a person acts in a given situation, depending on the environment and the persons with whom he or she is interacting in that moment. Such an action or reaction is dependent on a multitude of cognitive and emotional lexicons like information from our semantic memory - including moral knowledge - or our pattern of preferences. Also, the pattern of reactivity (impulsivity or apathy) as well as the eventual automatic routine behaviour a person is used to (compulsions, addictions, etc.) influence the individual way of reacting. Thus, behaviour is intimately related to the person's cognitive and affective status, being affected by brain pathology. The mechanisms bywhich brain lesions can affect a patient's behaviour depend on the pathogeny of lesions but also their localisation. There are different causes by which behavioural response can be affected in neurological diseases, and these causes correspond to the different steps which will lead to the patient's reaction. First, a neurological disease can alter patients' references and preferences, like in the case of taste modifications, religious and moral changes after brain lesions or even alteration of sexual habits. Changes in such values can lead to a different response or even an absence of conduct after a stimulus which produced attraction or repulsion before. Such modifications of values and preferences have repeatedly been described after anterior and limbic lesions, in diseases such as stroke, brain injury or epilepsy. Second, the intensity of the reaction can be levelled (apathy, athymhormia) or abnormally strong (impulsivity, aggression), leading to acquired sociopathy or borderline personality disorders, for example. Such quantitative modifications in reaction have particularly been reported after frontobasal focal lesions (e.g. traumatic brain injury or stroke) but also in dysfunction of subcortical structures, such as in pallidal ischaemia for athymhormia or Tourette's syndrome for impulsivity. Third, a normally expected reaction could be disturbed by an overwhelming reaction, which at the same time prevents any constructed and adapted behaviour; this is the case of addiction and obsessive compulsive disorders. Interestingly, these stereotyped behaviours are not found in an isolated fashion but are often associated with either other conduct modification (e.g. athymhormia) or cognitive difficulties (e.g. encoding or retrieval difficulties). Finally, the monitoring system of behaviour can be impaired, like in the case of anosognosia. Anosognosia can either disrupt the perception of the deficit (ignoring hemiplegia in AntonBabinski's syndrome or not remarking visual difficulties in cortical blindness) or the perception of the inappropriateness of conduct (e.g. the unawareness related to frontal lobe syndrome). Anosognosia is not only frequent in brain lesions but is also a tremendous hindrance to successful rehabilitation.