Hydrocephalus is the abnormal enlargement of cerebral ventricles and/or subarachnoid space as a result of excess cerebrospinal fluid (CSF) accumulation. There are two types of true hydrocephalus: communicating hydrocephalus, which is caused by decreased CSF resorption or increased CSF production in the absence of CSF flow obstruction, and noncommunicating hydrocephalus, which is caused by the obstructed passage of CSF from the ventricles to the subarachnoid space. Both forms typically manifest with signs of elevated intracranial pressure (ICP). Other clinical manifestations may include changes in vital signs resulting from brainstem compression and, in congenital hydrocephalus, macrocephaly. Normal pressure hydrocephalus (NPH) is a gradual-onset chronic form of communicating hydrocephalus that primarily occurs in adults > 60 years of age. ICP is normal or mildly elevated; manifestations include a classic triad of gait apraxia, dementia, and urinary incontinence. All types of hydrocephalus are diagnosed using MRI brain or CT head; ultrasound is used in children < 18 months of age with an open anterior fontanelle. A lumbar puncture may be required (once intracranial lesions with mass effect have been ruled out) to measure opening pressure, obtain CSF samples for analysis, and, if NPH is suspected, perform a CSF tap test. Temporizing measures to reduce CSF volume, such as external ventricular drain placement or serial lumbar punctures, may be used in the acute setting. Definitive treatment often involves surgical insertion of a shunt that drains excess CSF into another area of the body. Hydrocephalus ex vacuo is the enlargement of the ventricles and subarachnoid space due to loss of brain tissue (e.g., cerebral atrophy). It is not considered a true hydrocephalus because ventricular enlargement does not result from CSF accumulation and it does not affect ICP or CSF flow.