Disorders of consciousness present intriguing challenges to the neurologist and neurorehabilitation specialist. Assessment is constrained by the lack of reliable methods of assessing consciousness, and there are no treatment interventions known to influence the course of recovery from these conditions. In addition, the relationship between the clinical features associated with these disorders and their corresponding pathophysiologic substrate is also unclear. Our understanding of disorders of consciousness has not kept pace with the advances in neurosurgical management that have decreased mortality following severe injury. There is still considerable confusion regarding differential diagnosis and prognostication concerning states of severely altered consciousness. The purpose of this article is to discuss the content and neural basis of consciousness and to review the terminology most often used to describe altered states of consciousness. The neurobehavioral criteria for differentiating among specific syndromes associated with severe alterations in consciousness are presented. Representative case studies are utilized to illustrate the characteristic clinical profiles of coma, vegetative state, persistent and permanent vegetative state, minimally conscious state, akinetic mutism, and locked-in syndrome. Areas of ambiguity and controversy are emphasized and future directions for research are suggested.