Among patients in the emergency department (ED), most severe, sudden-onset headaches are primary, such as migraine or tension-type headache. Only 10% to 15% of patients have serious underlying pathology. However, guidelines for evaluation of patients with severe headache emphasize detection of subarachnoid hemorrhage (SAH) and other cerebrovascular conditions. Reversible cerebral vasoconstriction syndrome (RCVS) often is unrecognized and may be as common as SAH in patients with severe, sudden-onset headache in the ED. Evaluation of patients with severe headache focuses on obtaining a description of the episode and investigating any risk factors, clinical features, and red flag signs and symptoms that could indicate the presence of a serious condition. Any abnormal finding on neurologic examination has an approximately 39% positive predictive value for intracranial pathology. Computed tomography (CT)scan without contrast within 6 hours of symptom onset is highly sensitive for SAH. Lumbar puncture is recommended for patients with late presentation of SAH and select patients with initially negative CT scan results. CT or magnetic resonance angiography of the brain that shows multiple focal areas of vasoconstriction is diagnostic of RCVS. Lumbar puncture is indicated for patients with suspected meningitis. The management, follow-up, and prognosis of patients with severe headache depend on the etiology.