We will review positron emission tomography studies in comatose, vegetative, minimally conscious, locked-in and brain-death patients Coma is defined as the absence of arousal and hence of awareness. The vegetative state is a unique condition wherein all aspects of awareness are abolished whereas arousal is preserved. Patients in a minimally conscious state may show limited emotional or motor behavior that is contingent upon the presence of eliciting stimuli (e.g., crying precipitated by particular family voices). The defining feature of the locked-in syndrome is the complete preservation of cognition in a completely paralyzed body (small blinks or vertical eye movements are the only way to communicate). Functional neuroimaging permits to objectively measure how deviant from normal is the cerebral activity and its regional distribution, at rest and under various conditions of stimulation (e.g., auditory, noxious or emotional processing) in each of these different pathological states of (seemingly) altered consciousness.