Summary: THE appearance of a new book on radioactivity by Lord Rutherford and his associates is a noteworthy event in the world of physics. Rather than prepare a new edition of his standard treatise on “Radioactive Substances and their Radiations”? the senior author has chosen to devote his present discussion primarily to the significant developments in radioactivity which have occurred since that treatise appeared. These developments deal primarily with the characteristics of the radiations from radioactive materials and the effects which they produce. In view of the present state of physics, and the fundamental questions associated with the outer part of the atom apparently well understood, and the atomic nucleus offering perhaps the most vital problem with which physics is now faced, we are fortunate to be presented with this clear and comprehensive account of the information regarding the nucleus which is afforded by studies in radioactivity. Nothing could be more timely.