“Thunderclap headaches” are severe intensity headaches that reach maximum intensity in less than 1 minute. There are numerous etiologies of thunderclap headache, some associated with substantial morbidity and mortality and others with benign outcomes. Evaluation of the patient with thunderclap headache must occur urgently in order to assess for dangerous etiologies such as subarachnoid hemorrhage. When a cause for thunderclap headache is not identified after initial testing that includes brain computed tomography and cerebrospinal fluid evaluation, additional testing is typically indicated to determine the etiology. “Primary thunderclap headache” is diagnosed when a complete evaluation fails to identify a specific cause for thunderclap headache.