The causes and consequences of severe traumatic brain injury are reviewed, including mechanical injury and neurochemical changes following focal and diffuse injuries. A discussion of terminology seeks to clarify current nomenclature and descriptions of behaviours during emergence from coma. Theories of recovery following severe brain injury are reviewed including diaschisis, plasticity and compensation, redundancy, environmental regulation, and excess disability. Several assessment tools that have been recommended for use with this population of individuals in an altered state of consciousness following coma are discussed and critiqued. They include the Glasgow Coma Scale, Ranchos Los Amigos Levels of Cognitive Functioning Assessment Scale, Western Neuro Sensory Stimulation Profile, Coma Near Coma Scale, Freeman Questionnaire, and the Sensory Modality Assessment Rehabilitation Technique.