Subarachnoid hemorrhage (SAH) is a neurologic emergency due to bleeding into the subarachnoid space. Mortality can reach 50%. The clinical presentation is most often in the form of headache, classically defined as maximal at onset and worst of life. The most common cause is traumatic; approximately 80% of nontraumatic SAH are due to aneurysmal rupture, with the remainder from idiopathic peri-mesencephalic hemorrhage or other less common causes. Noncontrast brain computed tomography (CT) performed within 6 hours of symptom onset has sensitivity approaching 100%. Lumbar puncture may be considered after this period for definitive diagnosis if initial CT is normal.