New onset of sudden or progressive headache can have various causes, including disorders of intracranial pressure (ICP). Headache is the most common-and often the presenting-symptom of both intracranial hypertension and intracranial hypotension syndromes, which can be symptomatic or idiopathic. Despite the widespread availability of diagnostic tests, including ocular ophthalmoscopy, neuroimaging, and measurement of CSF pressure, delays in diagnosis or misdiagnosis of idiopathic intracranial hypertension and spontaneous intracranial hypotension remain common. If left untreated, idiopathic intracranial hypertension and spontaneous intracranial hypotension produce highly disabling headaches, and threaten vision, hearing, and in rare cases, brain function and life. To improve the diagnosis of idiopathic intracranial hypertension and spontaneous intracranial hypotension, changes in the overall diagnostic strategy for headaches will be necessary in most care centres. Improved understanding of CSF physiology and the mechanisms of idiopathic intracranial hypertension and spontaneous intracranial hypotension will guide the development of new treatments.