Although philosophers and cognitive neuroscientists have struggled to define human consciousness, physicians can identify and assess its two clinical dimensions: wakefulness and awareness. A comatose patient has neither wakefulness nor awareness; a patient in a vegetative state has wakefulness without awareness; and a minimally conscious patient has both, but awareness is impaired. Syndromes of unconsciousness have established diagnostic criteria, but they encompass a spectrum of severity of brain damage and have indistinct boundaries. Functional neuroimaging using PET and fMRI have provided a new and complementary way to assess consciousness. Several recent provocative studies suggest that fMRI in unresponsive patients may detect evidence of conscious awareness when a careful neurological examination cannot. If these findings are verified by future studies, functional neuroimaging technologies will alter clinical practices concerning the diagnosis, classification, and prognosis of unconscious patients, and will lead to a greater understanding of the biology of human consciousness.