Headaches are one of the most common complaints of patients seen by emergency physicians. They can be classified as primary headaches, which have no identifiable underlying cause, and secondary headaches, which are classified according to their cause. The majority of headaches are benign in origin, and most patients with headache can be treated successfully in the emergency department and discharged home; however, some have potentially life-threatening causes, and consideration of a broad differential diagnosis for all patients is essential. This review covers the primary headache disorders, pathophysiology, stabilization and assessment, diagnosis and treatment, and disposition and outcomes. The figure shows areas of the brain sensitive to pain. Tables review differential diagnosis of headache, International Headache Society primary headache criteria, clinical characteristics of secondary headaches, high-risk clinical characteristics among patients with a headache peaking in intensity within 1 hour, drugs associated with headache, and parenteral treatment of acute migraine. This review contains 1 figure, 9 tables, and 58 references. Key words: migraine, calcitonin gene related peptide, greater occipital nerve block, venous sinus thrombosis, reversible cerebral vasoconstriction syndrome, Ottawa, subarachnoid, cluster headache, trigeminal autonomic cephalalgias, post-traumatic headache