Stored-program-controlled exchanges which, because of their flexibility, are generally preferred for new systems, have economic difficulties of application in practice. In a general review of the problems involved, the design of electronic exchanges is analysed, and the flexibility of the various kinds of control is assessed. It is shown that flexibility of call establishment is flexibility not of processing but of information storage, which is realisable in all systems. In respect of changes of supervision during the lifetime of an exchange, stored-program-controlled systems have an advantage over other systems which has to be assessed as cost saving during the lifetime of the exchange against cost at the initial installation. On this basis, it seems that the amount by which the capital cost of stored-program-controlled exchanges could exceed that of others is small. A system of exchanges designed for wired-logic control and for information storage which is controllable from a distance, and in which, depending on the application, wired logic is replaced by stored program to an extent determined by economy, may be the best way of relising in one system the advantages of both forms of control. A system on this basis is described in companion papers.