AbstractIn reactive processes a process gas is fed into the process chamber after evacuation. The most important precondition for a reproceable process is a stable process pressure.Pressure control is carried out either by controlling the gas flow on the gas inlet side or by regulating the effective pumping speed on the chamber output side.The application of the user determines, which of the two sysstms is best suited.Plasma enhanced etching and coating processes encounter a growing importance in the semiconductor industry. At the same time applications have expanded to improvement procedures of metallic surfaces and production of computer hard discs, based on plasma processes as reactive vapour deposition and sputtering.For many of these applications pressure control via the gas input is sufficient. But the increasing demands on the quality of the micro structures with modern surface processes can only be satisfied by independent control of the gas flow and the process pressure. Pressure control independent of the gas flow is achieved by controlling the effective pumping speed of the vacuum pump.The presented paper starts with the general theory of pressure control, and deals with the various aspects of pressure control, including downstream pressure control with throttle valvê, adaptive controller and sensor