Hydrocephalus is caused by different etiological factors, but the common final pathway creates a vicious circle of altered cerebrospinal fluid (CSF) circulation, cerebral blood flow, and metabolism that affects brain homeostasis. In communicating hydrocephalus, draining of CSF appears to be the simplest way of interrupting this circle by improving the CSF circulation. The mathematical model of CSF pressure volume compensation provides a theoretical basis for the differential diagnosis of hydrocephalus. Eighteen nonprogrammable and eight programmable valves reveal the common hydrodynamic properties of contemporary shunts. Any repetitive variations of proximal pressure have a tendency to decrease the nominal operating pressure of shunts with unidirectional valves. Shunt operating pressure is the value of intracranial pressure (ICP) that should be fixed when the opened shunt is draining CSF with the rate equal to that of its production. It is slightly higher than shunt opening pressure.