This review discusses the nosological criteria and functional neuroanatomical basis for brain death, coma, vegetative state, minimally conscious state, and locked-in syndrome. Functional neuroimaging techniques, such as PET, fMRI, and electrophysiological methods, offer new insights into cerebral activity in patients with severe brain damage. These techniques can provide unique windows into the presence, degree, and location of residual brain function. However, their use in severely brain-damaged patients is methodologically complex and requires careful quantitative analysis and interpretation. Ethical frameworks to guide research in these patient populations must be further developed. Current clinical assessments remain the standard for accurate diagnosis and prognosis. Neuroimaging techniques, while promising, are important tools for clinical research that should ultimately enhance our understanding of these disorders. The review defines consciousness as it can be assessed at the patient's bedside and discusses the major clinical entities of altered states of consciousness following severe brain damage. It also discusses recent functional neuroimaging findings in these conditions, with a special emphasis on vegetative state patients. Consciousness is a multifaceted concept involving the level of consciousness (arousal, wakefulness, vigilance) and the content of consciousness (awareness of the environment and self). Arousal is supported by brainstem neuronal populations, while awareness depends on the functional integrity of the cerebral cortex and its subcortical connections. Brain death is defined as the irreversible loss of brainstem function. Coma is characterized by the absence of arousal and consciousness, while the vegetative state involves being awake but unaware of self or environment. The minimally conscious state involves limited but discernible evidence of consciousness, and locked-in syndrome involves quadriplegia and anarthria due to disruption of corticospinal and corticobulbar pathways. Functional neuroimaging studies show that in vegetative patients, brain metabolism is significantly reduced, with preserved metabolism in certain brain regions. In the minimally conscious state, there is more widespread activation of brain regions compared to vegetative patients. In locked-in syndrome, there is preserved awareness but limited motor function. The review also discusses methodological issues in neuroimaging studies, including the complexity of data acquisition, analysis, and interpretation. Ethical concerns regarding the participation of severely brain-damaged patients in neuroimaging studies are also addressed. The review concludes that while neuroimaging techniques offer exciting new opportunities in the assessment of severely brain-damaged patients, they are methodologically complex and subject to multiple difficulties of analysis and interpretation. Functional imaging should complement, rather than replace, standardized clinical evaluations.This review discusses the nosological criteria and functional neuroanatomical basis for brain death, coma, vegetative state, minimally conscious state, and locked-in syndrome. Functional neuroimaging techniques, such as PET, fMRI, and electrophysiological methods, offer new insights into cerebral activity in patients with severe brain damage. These techniques can provide unique windows into the presence, degree, and location of residual brain function. However, their use in severely brain-damaged patients is methodologically complex and requires careful quantitative analysis and interpretation. Ethical frameworks to guide research in these patient populations must be further developed. Current clinical assessments remain the standard for accurate diagnosis and prognosis. Neuroimaging techniques, while promising, are important tools for clinical research that should ultimately enhance our understanding of these disorders. The review defines consciousness as it can be assessed at the patient's bedside and discusses the major clinical entities of altered states of consciousness following severe brain damage. It also discusses recent functional neuroimaging findings in these conditions, with a special emphasis on vegetative state patients. Consciousness is a multifaceted concept involving the level of consciousness (arousal, wakefulness, vigilance) and the content of consciousness (awareness of the environment and self). Arousal is supported by brainstem neuronal populations, while awareness depends on the functional integrity of the cerebral cortex and its subcortical connections. Brain death is defined as the irreversible loss of brainstem function. Coma is characterized by the absence of arousal and consciousness, while the vegetative state involves being awake but unaware of self or environment. The minimally conscious state involves limited but discernible evidence of consciousness, and locked-in syndrome involves quadriplegia and anarthria due to disruption of corticospinal and corticobulbar pathways. Functional neuroimaging studies show that in vegetative patients, brain metabolism is significantly reduced, with preserved metabolism in certain brain regions. In the minimally conscious state, there is more widespread activation of brain regions compared to vegetative patients. In locked-in syndrome, there is preserved awareness but limited motor function. The review also discusses methodological issues in neuroimaging studies, including the complexity of data acquisition, analysis, and interpretation. Ethical concerns regarding the participation of severely brain-damaged patients in neuroimaging studies are also addressed. The review concludes that while neuroimaging techniques offer exciting new opportunities in the assessment of severely brain-damaged patients, they are methodologically complex and subject to multiple difficulties of analysis and interpretation. Functional imaging should complement, rather than replace, standardized clinical evaluations.