Brainstem stroke due to an embolus to the basilar artery, is one of the most common causes of initially unexplained coma. This chapter explains why and how patients become comatose after an ischemic or hemorrhagic stroke. Acute coma from a stroke may be due to an embolus to the basilar artery destroying the ascending reticular formation in the dorsal pons and mesencephalon. The approach to a patient with coma and abnormal consciousness requires three steps. First, determine whether the patient is truly comatose and exclude confounders or misleading signs. Second, with a focused neurological examination, it is possible to localize the lesion using information on brainstem reflexes and motor responses to noxious stimuli. Third, breathing patterns, blood pressure, pulse, and temperature characteristics may also indicate a certain cause. Coma is expected in catastrophic and often fatal ganglionic cerebral or pontine hemorrhages.