A wide spectrum of disorders, ranging from intracranial hemorrhage to hypoglycemia, present with impaired consciousness as a leading symptom. The duration of these periods of impaired consciousness varies from less than a minute (for example, in syncope) to days and weeks at a time, with certain patients confined to a lifetime of disordered consciousness in states such as minimally conscious state (MCS) and persistent vegetative state (PVS). As improvements in medical science and technology increase the survival of patients in states of impaired consciousness, the problem of properly differentiating between these states and recognizing their causes early on steadily grows in importance. The basis for establishing a correct diagnosis of these disorders is, perhaps even more so than in other areas of clinical neurology, a strong grasp of neuroanatomy and its skillful application in the neurological examination. The ascending reticular activating system (ARAS) of the brainstem, the main pathway regulating wakefulness and sleep, lies in close proximity to certain nuclei whose function can be clinically tested even in unconscious patients, allowing for localization of a potential focal lesion and assessment of damage. Neuroimaging, laboratory testing and electrophysiological studies can further identify different levels and causes of consciousness disorders. This paper aims to review these potential causes, the pathophysiology of the damage they cause and the specificities in their manifestation that can be used to differentiate between them. Determining the complete absence of brain activity, also known as “brain death”, is an increasingly important issue, especially relevant in transplantation medicine.