The object of this paper is to give a description of the methods employed by the British Post Office in making telephone development studies, framing lay-out schemes and determining the economic period for which to provide plant. As the paper is to be read in conjunction with another paper * dealing with the methods employed in power systems, detailed descriptions which would only be of interest to telephone engineers have been avoided. It has thus been possible to cover a great deal of ground; but certain matters, each of which would require much space if it were dealt with adequately, have had to be dealt with in a few paragraphs. Nevertheless, it is thought that, with the information contained herein, a fair idea can be obtained of the methods adopted in the Post Office in making the studies described in the title of this paper