An instrumentation system or a control system does not have to exist at one specific location, nor does a system have to exist in a specific cabinet. Nowadays, the idea of centralised systems is being replaced by the concept of distributed systems. Control engineers long ago discovered the frailty of centralised control rooms and took the advent of the microprocessor to distribute control actions to the point of application. Instrumentation engineers working in the same epoch seized the opportunity to incorporate intelligence into the point of measurement, and computer engineers developed the technique of distributed computing by means of networking. This chapter investigates the means of communication between instruments and controllers using digital computers. It deals first with the ideas of distributed systems and discusses serial interfaces. Later, the modern techniques of local and wide area networking are applied, but not in detail.