Telecommunication systems depend for much of their effectiveness on a high degree of standardisation on a worldwide basis, so that calls can be made from virtually every subscriber in the world to any other. Although there is competition in the supply of equipment, the basic principles of collaborative interworking remain mandatory. In computers there are often only minimal areas of comparable standardisation. Even high-level languages are often particular to a machine or a manufacturer, and computer users are frequently unable to run programs written for one machine on a different machine from the same maker. A high degree of mutual dependence is now coming into existence between telecommunication systems and computing systems. Examples are now widespread of computer-like machines being used to control or assist communications plant, and of telecommunication networks being used to link computers together. In addition, a substantial possibility exists of technology developed for computers, often leaders in electronics techniques, being applied to communications uses. Moreover, the ability of even modest telecommunication systems to maintain reasonable service in the presence of faults suggests that this ‘system’ level of organising reliability could with advantage be incorporated into some computing systems.