This paper is concerned with the selection of optimum channels for communication through media of which the quality is varying from any cause, including changing levels of interference. In particular it demonstrates the advantage of adaptive frequency selection when a block of channels is shared by relatively large numbers of random intermittent users, as is now the case with many portions of the h.f. band. Various methods of channelling including the broad-band spread-spectrum approach are discussed, and it is shown that adaptive selection of narrow band channels could be compatible with existing non-adaptive usage. The argument is further extended to include a discussion of adaptive use of the v.h.f. and u.h.f. bands for satellite communication on the basis of noninterference with existing services.