The preliminary section of the paper deals with the early history of research on selenium, and its sensitivity to light. In Section 1, dealing with the construction of selenium cells, different types of cells are enumerated and described. It is emphasized that the cell must have little inertia to light-changes, and for this reason the selenium layer must be thin so that as much as possible is affected by the light. The properties of selenium cells are dealt with in Section 2. The author's experiments have shown that the change in conductanceG(due to a given illuminationE) varies as some power of the illuminationE, i.e.G∞Ex. The index valuexvaries from type to type of cell, and even amongst cells of the same type small variations inxoccur. Thus no two cells, even of the same type, have in general exactly the same quantitative reaction towards a given illumination. It is shown, however, that these empirical relations are quantitatively reproducible to a high degree of accuracy, provided the experimental conditions are reproduced.A brief résumé of the many factors influencing the conductance of a selenium cell is followed in each case by the results of recently published research. Some experiments by the author on the decay of the conductance of selenium after exposure to illumination show that the internal state of the selenium, as determined by the change in conductance, is, throughout a large portion of the visible spectrum, independent of the quality of the stimulus. Some filter experiments are described, from which it appears that the change in conductance of selenium cells is dependent, not on the number of foot-candles incident on the cell, but rather on the amount of radiant energy received. Section 2 concludes with a short note on the properties of selenium crystals.The chemical, electrolytic and electronic theories of the action of light on selenium are reviewed in Section 3. The recent researches of Gudden and Pohl suggest a return to the physico-chemical theory. It appears that the problem of explaining the observed phenomena is not one of photo-electricity but rather of conductivity in non-homogeneous media containing a large number of bounding crystal surfaces.In Section 4 the practical applications of selenium cells are described in some detail under the following headings:—(a) Application of the selenium cell to photometry, and to relay problems.(b) The Optophone.(c) The Photophone.(d) The talking film.(e) Television.The conclusion sums up the present state of our knowledge of the action of light on selenium.