For some years the Authors' company has engaged in the design and manufacture of closed-loop hydraulic control systems. The hydraulics form a high performance system of considerable power. But the control is exercised electrically through the agency of a servo valve. This note describes the development of an electronic system for this application. Although originally rather specialised in purpose, the system has in the course of development become universal. Advantage has been taken of recent improvements in electronic techniques and at the time of writing the system is as advanced as any. Nonetheless, the speed at which electronics develops nowadays is such that some parts of the system may get outdated, and superseded. It has already happened once in the course of the work that a major invention the Integrated Circuit — has made obsolete an entire concept using discrete components before the development was complete. The same can happen again.