Approximately one-half of the adult population worldwide is affected by a headache disorder. The International Headache Society classification and diagnostic criteria can help physicians differentiate primary headaches (e.g., tension, migraine, cluster) from secondary headaches (e.g., those caused by infection or vascular disease). A thorough history and physical examination, and an understanding of the typical features of primary headaches, can reduce the need for neuroimaging, lumbar puncture, or other studies. Some red flag signs and symptoms identified in the history or during a physical examination can indicate serious underlying pathology and will require neuroimaging or other testing to evaluate the cause of headache. Red flag signs and symptoms include focal neurologic signs, papilledema, neck stiffness, an immunocompromised state, sudden onset of the worst headache in the patient's life, personality changes, headache after trauma, and headache that is worse with exercise. If an intracranial hemorrhage is suspected, head computed tomography without contrast media is recommended. For most other dangerous causes of headache, magnetic resonance imaging or computed tomography is acceptable.