The failure to recognize the existence of disease is known as anosognosia. This article provides a brief discussion of the evolution of this concept and reviews some qualitative differences in the manner in which the disavowal of neurogenic impairment is expressed. Theoretical explanations for the unawareness of deficit after neurologic illness include motivational-symbolic, cognitive subsystem, and supraordinate system theories. Observations from a clinically derived, structured awareness interview are presented, suggesting three factors that may underlie patients' apparent lack of awareness of deficits after traumatic brain injury. These include diminished awareness of deficits secondary to impaired cognition, especially memory and reasoning deficits; psychological reactance and denial of deficits; and a relatively "pure" inability to recognize areas of impaired functioning as a direct consequence of brain injury. The causes of unawareness are likely to be complex and multiply determined in any given patient, although it may be possible to identify primary, secondary, and even tertiary contributions according to specific behavioral and phenomenological characteristics. The ability of patients to modify their perceptions and acknowledgment of deficits after objective feedback may have particular diagnostic value and clinical utility in this regard.