Acute headache is a common symptom and is reported by approximately 2%-4% of patients who present to the emergency department. Many abnormalities manifest with headache as the first symptom, and it is crucial to obtain a patient's complete clinical history for correct diagnosis. Headache onset, duration, and severity; risk factors such as hypertension, immunosuppression, or malignancy; and the presence of focal neurologic deficits or systemic symptoms may aid the radiologist in deciding whether imaging is appropriate and which modality to choose. Imaging findings are more likely to be abnormal in patients with a "thunderclap" headache than in those with headaches of lesser severity. The causes of headache in the emergency setting are various. They may manifest at imaging as subarachnoid hemorrhage (ruptured aneurysm, reversible vasoconstriction syndrome, or pituitary apoplexy), parenchymal hemorrhage (hypertension, ruptured arteriovenous malformation, cerebral amyloid angiopathy, dural arteriovenous fistula, or sinus thrombosis), or parenchymal edema (posterior reversible encephalopathy syndrome, reversible cerebral vasoconstriction syndrome, sinus thrombosis, or encephalitis). Alterations in intracranial pressure that are related to idiopathic intracranial hypertension or spontaneous intracranial hypotension and prior lumbar puncture or epidural injection may manifest with specific imaging findings. With accumulating knowledge of disease pathophysiology, radiologists have started to play a more central role in making the correct diagnosis. This article reviews multiple causes of acute headache and their characteristic appearances at multimodality imaging and familiarizes the reader with current concepts in imaging.Online supplemental material is available for this article.©RSNA, 2019.