There are a number of dangerous secondary causes of headaches that are life, limb, brain, or vision threatening that emergency physicians must consider in patients presenting with acute headache. Careful history and physical examination targeted at these important secondary causes of headache will help to avoid misdiagnosis in these patients. Patients with acute thunderclap headache have a differential diagnosis beyond subarachnoid hemorrhage. Considering the “context” of headache “PLUS” some other symptom or sign is one strategy to help focus the differential diagnosis.