This chapter lists out the spectrum of abnormal levels of consciousness, and neurological conditions imitating coma that include vegetative state, minimally conscious state, locked-in syndrome, and brainstem death. The causes of unconsciousness are categorized into diffuse cortical impairment, direct injury and injury due to pressure effects. Immediate assessment of airway patency, cervical spine stability (in cases of trauma), breathing and circulation should be performed in unconscious patient. The level of consciousness is assessed by using either Glasgow Coma Scale (GCS) or alert, voice, pain, unresponsive (AVPU) scoring system, and pupillary examination. The chapter lists out different investigation methods that include laboratory tests, radiological tests, electroencephalography and lumbar puncture. The fundamental goal in management of unconscious patients should be to limit further damage to the brain and treat the underlying cause. The management of unconscious patients is categorized into immediate management and long-term management.