By Arthur A. Vuylsteke London: D. Van Nostrand Co. Ltd. Pp. xiii + 362. Price 71s. 6d. In his preface to this comprehensive book the author points out that in "the relatively new science of Quantum Electronics which has as its primary aim the practical utilization of the quantum behaviour of matter ... it is necessary to combine, in a more obvious fashion than has heretofore been apparent, the principles and techniques of a number of allied but distinct disciplines ". To define and map out the ground which the practitioners of these " allied but distinct disciplines " must make common, Dr. Vuylsteke has written this volume " in which the necessary, basic physical theories are presented as an introduction to a rather complete discussion of that part of maser theory which is most basic and which is presently well understood ".