Vary depending on size, location, and nature of<unk> 
Features
Sudden onset or accelerating pattern
Maximum intensity of pain at onset (i.e. "thunderclap")
Worse with valsalva
Worse in the morning or at night
No similar headache in past
Age >50 yr or <5 yr
Occipitonuchal headache
Visual disturbances
Exertional or postcoital
Family or personal history of SAH, cerebral aneurysm, or AVM
Focal neurologic signs
Diastolic BP >120
Papilledema
Jaw claudication
Clinical Context
Headache in setting of:
Infection
Cancer
Immunosuppression
Seizure
Syncope
Trauma
Altered mental status
Systemic illness (fever, stiff neck, rash)
Nausea/vomiting
Patient on anticoagulation, steroids, NSAIDs, antiplatelet
Intracranial mass
Intracranial hemorrhage
Epidural hemorrhage
Intra-axial
Epidural abscess 
Hemorrhagic stroke 
Post-lumbar puncture headache
Traumatic intracerebral hemorrhage