This review considers the production and absorption of cerebrospinal fluid (CSF) as they relate to intracranial pressure (ICP) in normal individuals and patients with hydrocephalus. It also considers the physics of various types of shunts used to treat hydrocephalus. The hope is that understanding normal and abnormal CSF flow and the physics of shunts will help physicians select the valve mechanism best suited for individual patients with hydrocephalus. CSF formation is constant over a wide range of ICP. In contrast, CSF absorption depends on ICP. There is no drainage if ICP is less than 5 mm Hg, and the rate of absorption is directly proportional to ICP after the opening pressure is reached. The natural valve is a ventricular-sagittal sinus shunt with an opening pressure of 70 mm H2O. More complicated strategies must be considered because of the effects of siphoning related to the routine use of the peritoneum for the treatment of hydrocephalus. The physics of valve designs commonly used in the United States, as reflected by their pressure-flow characteristics, are described. Hard data from randomized control trials on which to base the selection of one valve compared with another are unavailable. However, specific strategies should be considered in a few conditions, such as posthemorrhagic hydrocephalus in the premature newborn in whom valves should be selected based on their ability to drain CSF with high protein and particulate matter and without the need to compensate for siphoning in the small infant. Patients with normal pressure hydrocephalus need valves that have a low opening pressure but that do not lead to extreme low pressure due to siphoning when an erect position is assumed. Finally, patients symptomatic from overdrainage require high-pressure shunts with devices that retard siphoning; they may benefit from valved lumboperitoneal shunts.