Thunderclap headache is a severe and acute headache that reaches maximum intensity in under one minute and lasts for more than five minutes. Subarachnoid haemorrhage (SAH) accounts for 10-25% of all thunderclap headaches and, despite advances in medical technology, has a 90-day mortality of 30%. Up to a quarter of cases of SAH are misdiagnosed, often through failure to follow guidance. Thunderclap headaches may be associated with symptoms such as photophobia, nausea, vomiting, neck pain, focal neurological symptoms or loss of consciousness. SAH is more likely if there are neurological abnormalities or reduced consciousness. Loss of consciousness at onset is a poor prognostic indicator with a 2.8-fold increase in risk of death. All patients with suspected SAH should undergo a non-contrast CT brain scan as soon as possible after the onset of pain as the sensitivity of CT drops with time. A negative CT is not sensitive enough to exclude SAH and must be followed with lumbar puncture at least 12 hours after onset of the headache. If SAH is excluded then further investigations, in particular MRI brain and vascular imaging with MRI or CT angiography, should be considered to exclude other aetiologies. Headaches, caused by cervical artery dissection are most commonly of gradual onset but up to 20% of patients complain of thunderclap headache.