Introduction: Thunderclap headache (TCH) is an excruciating headache that reaches maximal intensity within a minute. It has numerous potential etiologies, the most concerning of which is subarachnoid hemorrhage (SAH) due to high morbidity and mortality. Thus, patients with TCH must be evaluated urgently to identify the underlying cause and initiate prompt therapy. Areas covered: This paper reviews PubMed-listed research articles and presents an update of the clinical features, diagnostic evaluation, and possible causes of TCH. Expert commentary: In addition to SAH, TCH has been associated with reversible cerebral vasoconstriction syndrome (RCVS), cervical artery dissection, cerebral venous sinus thrombosis, cerebral infarction, intracerebral hemorrhage, spontaneous intracranial hypotension, intracranial infection, and pituitary apoplexy. Of note, with advances in knowledge in the past decade, RCVS has become an important cause of TCH, being diagnosed more frequently. Brain computed tomography (CT) should be performed in all patients with TCH, and lumbar puncture is indicated if the brain CT is nondiagnostic. Generally, a negative brain CT and lumbar puncture can eliminate SAH diagnosis, in which case brain magnetic resonance imaging and vascular imaging should be performed to evaluate other possible underlying causes.