Damage to different regions of the brain can cause a variety of neuropsychological deficits, including specific disturbances of language, memory, perception and motor function. A significant number of brain-damaged patients are unaware of their deficits, even when they are profound and have debilitating effects on patients' performance. This article reviews clinical observations and experimental investigations concerning unawareness of deficits, considers methodological issues, and critically evaluates different interpretations of the phenomenon. An integrative theoretical framework is proposed to account for unawareness of deficits in diverse neuropsychological syndromes. Possible directions for future research are outlined.