An important class of filtration problems in telecommunication is associated with frequency changing; it includes the generation and demodulation of single-sideband carrier channels and the elimination of image-frequency interference in heterodyne demodulators, such as the superheterodyne radio receiver or the conventional wave analyser. Filters for these applications are often difficult to design or realize, or may be inconvenient on account of variable tuning, etc.It has been realized recently that polyphase modulation can be used as part of the frequency-changing process with great advantage. It can eliminate the need for difficult or inconvenient filters, although other design problems are introduced which may some times be as difficult to solve. But there is no doubt that the idea of polyphase modulation is of great importance in telecommunication theory and practice. The basis of the advantages given by polyphase working is that polyphase signals possess an identifying property additional to that of frequency, namely sequence. By using circuits which distinguish between signals of the same frequency but opposite sequence, it is possible, without any preliminary filtration, to separate signals which lie in the same frequency band after modulation, and which, therefore, could be separated by normal means only by filters before the modulation stage.The first Section of the paper outlines the main filtration problems which can be tackled by polyphase methods, and then the necessary polyphase theory is given. This is followed by a discussion of circuit design for polyphase modulation and sequence discrimination.The list of references shows that there have been many publications covering some of the separate applications of this work, but the paper is believed to present for the first time a comprehensive theory of polyphase modulation embracing all the known applications.