This clinical policy from the American College of Emergency Physicians is an update of a 2002 clinical policy on the evaluation and management of adult patients presenting to the emergency department (ED) with acute, nontraumatic headache. A writing subcommittee reviewed the literature to derive evidence-based recommendations to help clinicians answer the following 5 critical questions: (1) Does a response to therapy predict the etiology of an acute headache? (2) Which patients with headache require neuroimaging in the ED? (3) Does lumbar puncture need to be routinely performed on ED patients being worked up for nontraumatic subarachnoid hemorrhage whose noncontrast brain computed tomography (CT) scans are interpreted as normal? (4) In which adult patients with a complaint of headache can a lumbar puncture be safely performed without a neuroimaging study? (5) Is there a need for further emergent diagnostic imaging in the patient with sudden-onset, severe headache who has negative findings in both CT and lumbar puncture? Evidence was graded and recommendations were given based on the strength of the available data in the medical literature.