Idiopathic intracranial hypertension (IIH) is a rare disorder that typically affects obese women of childbearing age, but can also occur in paediatric populations. Patients usually present with diffuse, daily headache and visual disturbances, but either symptom can occur in isolation. Patients with IIH often have papilloedema; however, IIH without papilloedema is fairly common in patients with chronic daily headache. The pathogenesis of IIH is unknown; the high incidence of comorbid bilateral transverse sinus stenosis (BTSS) in patients with IIH suggests that the two conditions are linked, although no direct causal relationship has been established. Cerebrospinal fluid (CSF) pressure monitoring or lumbar puncture-which provides immediate symptomatic relief-are important in making a diagnosis of IIH. Current treatments for IIH include weight reduction, medical treatment, CSF diversion surgery, optic nerve sheath fenestration and, potentially, endovascular stenting (in patients with BTSS). Prevention of visual loss (which can be substantial) is the main goal of treatment. Residual headache and IIH recurrence are not uncommon after treatment, and regular follow-up is, therefore, warranted even in patients who achieve remission. This Review provides an update of current knowledge of the aetiology, pathophysiology and treatment of IIH.