Originally published in 1939, this book provided readers with a concise explanation of contemporary developments in the understanding of cosmic rays. During the two years preceding publication, the application of quantum theory to cosmic rays made it possible to give a connected account of a large part of the phenomena. The theory of radiation from a moving electron provided a satisfactory account of the behaviour of the less penetrating cosmic rays, and it became clear that the more penetrating cosmic rays could not be explained in terms of electrons, but required a new particle to account for them. The text is divided into two broad sections: the first chapters are devoted to the experimental facts of cosmic rays; the later to their explanation in terms of the properties predicted by quantum theory for moving charged particles.