This paper deals with the state of the short-wave practice and theory, and describes experiments made bythe author to elucidate short-wave transmission phenomena, and certain theoretical developments based on these experiments.In the first Section the aerial transmission characteristics are dealt with; in particular the vertical polar diagrams of transmission are computed, taking account of the earth's resistivity, and it is shown that these are all characterized by the absence of horizontal radiation.It is also shown that the intensity of the low-angle transmission is increased by raising the transmitting aerial.In the second Section the results of a series of experiments in which a short-wave radio direction-finder is used are described, and the conclusion is arrived at that in longdistance transmission the ray trajectory has a shallow angle of elevation 15°, and follows the great-circle path.In the third Section the results ofa series of long-distance transmission tests is given. Waves between 25 and 10 m were used, and experiments were made which indicated the superiority of a raised doublet over one at the ground-level.In the last Section the general theory of ionic refraction is discussed in the light of these and other experimental results. The effect of attenuation due to the collision of electrons with molecules is especially stressed and a novel theory of the skipdistance is put forward which has the merit of agreeing closely with the observed results of Heising, Schelleng and Southworth.