Summary:A brief taxonomy of neurologic disorders resulting in global impairments of consciousness is presented. Particular emphasis is placed on focal injuries of subcortical structures that may produce disorders that are otherwise associated to large bilateral cortical injuries. A distinction between subcortical arousal and "gating" systems is developed. Both clinical and experimental studies are reviewed in the context of these disorders and their possible underlying mechanisms. Key Words: Awareness-Persistent vegetative state-Intralaminar thalamic nuclei-Akinetic mutism-Reticular formation.Understanding the disorders of human consciousness provides an empirical first step toward understanding mechanisms underlying human awareness. The neurologist's view of consciousness is derived from reliable observations of brain structure and function that lead to implicit hypotheses. These hypotheses represent implied models that may be examined in light of many recent advances in clinical and experimental neuroscience. Impaired consciousness of a variety of kinds may result from focal brain injuries that induce widespread functional changes. Differences in the clinical features of these human disabilities illustrate the interdependent relationships of brain mechanisms underlying arousal, gating, and expression of several specific neuropsychologic traits. In many cases, brain damage is too severe to permit meaningful recovery, as exemplified by the vegetative state. In others, transient or permanent behavioral abnormalities precipitated by focal lesions may reveal the contribution of specific brain systems to the overall conscious state. Global disorders of consciousness include stupor and coma, vegetative states, akinetic mutism, absence and partial complex seizures, hyperkinetic mutism, and others. These disturbances can be acute, and relatively brief, or may persist as protracted states. The authors explore the neurology of impaired consciousness, developing a brief taxonomy of global disorders of consciousness, and placing these neurologic diseases in the context of the underlying anatomy and physiology of arousal and "gating" systems. Definition of consciousness: A definition closely following that of James (1890) is offered for the psychological dimension of consciousness:At its least, normal human consciousness consists of a serially time-ordered, organized, restricted and reflective awareness of self and the environment. Moreover, it is an experience of graded complexity and quantity.