This clinical policy focuses on critical issues in the evaluation and management of patients with acute headache. A MEDLINE search was performed, abstracts were reviewed, and appropriate full-text articles were read; references from reviewed articles were searched for additional material. This policy focuses on 4 areas of current interest and/or controversy in acute headache management: (1) response to headache therapy as an indicator of underlying pathology, (2) clinical findings predictive of increased intracranial pressure, (3)indications for emergent neuroimaging in patients with a complaint of headache, and (4) indications to pursue emergent diagnostic studies in patients with thunderclap headache but with normal findings on a head computed tomography (CT) scan and negative findings on a lumbar puncture. Recommendations for patient management are provided for each of these 4 topics based on strength of evidence. Level A recommendations represent patient management principles that reflect a high degree of clinical certainty, Level B recommendations represent patient management principles that reflect moderate clinical certainty, and Level C recommendations represent other patient management strategies based on preliminary, inconclusive, or conflicting evidence, or based on panel consensus. This guideline is intended for physicians working in hospital-based emergency departments.