Modern contactor design is reviewed and reference is made to the influence which various applications of contactor type control gear have had on design and development.After a general survey the special considerations appertaining to motor-control duty are examined in detail, having due regard to the extreme cases which frequently occur in practice. Arc control and contact developments are mentioned, and a short discussion of the oil-immersed contactor follows with particular reference to its use in chemical plants and for high voltage. Some special contactor designs are discussed, as well as the problems arising in connection with the control of electric furnaces. A short résumé of the desirable features of contactor auxiliary switches concludes the paper.