Apathy is commonly described following traumatic brain injury (TBI) and is associated with serious consequences, notably for patients' participation in rehabilitation, family life and later social reintegration. There is strong evidence in the literature of the multidimensional nature of apathy (behavioural, cognitive and emotional), but the processes underlying each dimension are still unclear. The purpose of this article is first, to provide a critical review of the current definitions and instruments used to measure apathy in neurological and psychiatric disorders, and second, to review the prevalence, characteristics, neuroanatomical correlates, relationships with other neurobehavioural disorders and mechanisms of apathy in the TBI population. In this context, we propose a new multidimensional framework that takes into account the various mechanisms at play in the facets of apathy, including not only cognitive factors, especially executive, but also affective factors (e.g., negative mood), motivational variables (e.g., anticipatory pleasure) and aspects related to personal identity (e.g., self-esteem). Future investigations that consider these various factors will help improve the understanding of apathy. This theoretical framework opens up relevant prospects for better clinical assessment and rehabilitation of these frequently described motivational disorders in patients with brain injury.