The first extensive application of the high-speed motor uniselector was in the mechanization of a system for handling long-distance telephone traffic in an overseas installation completed in 1940–41. Further development in this field was suspended during the war, although considerable use was made of the switch in equipment associated with the Air Defence of Great Britain. Since the war, the British Post Office has standardized the motor uniselector for trunk switching in the United Kingdom and, in addition, its use overseas, both in trunk and local systems, is rapidly extending.The paper describes the latest forms of the switch, those features of its design which ensure reliability and low maintenance costs, the facilities afforded by it, and the technique associated with its application in telephone switching and similar systems. A description is also given of the newly developed digit switch used with the motor uniselector.The standard circuit elements used in the control of the switch are dealt with, and an outline is given of its application as the basis of a mechanized trunk switching system and a local switching system. Reference is also made to its use in various miscellaneous applications.Attention is directed to the fact that the design of the switch and of the systems incorporating it is the logical outcome of a scientific study of the requirements to be met, not merely a re-development of traditional apparatus and arrangements.