DURING recent years the research on piezoelectric crystals has extended from quartz and Rochelle salt to cover many new water-soluble synthetic materials. Dr. W. P. Mason has contributed a large part to the new development and has compiled a lucidly written book based mainly on published and unpublished investigations of the Bell Telephone Laboratories. The emphasis in this book centres on the presentation of new results rather than on known subjects, but for completeness the properties and applications of quartz and Rochelle salt are included. The first chapters are written in the form of a general introduction to piezo-electricity and contain besides a survey of crystal systems, classes and symmetriesthe elastic, piezo-electric and dielectric relations in crystals, given in tensor notation. The extent of the search for new materials can be seen from the chapter entitled Properties of Crystals Determinable from Small Sizes, which contains a list of nearly two hundred crystals classified into four groups according to their coupling-strength. The thorough investigation culminated in the discovery of ethylene diamine tartrate and the useful properties of dipotassium tartrate, which are replacing quartz crystals in electrical filters. These materials have been treated in a separate chapter. The properties of ammonium dihydrogen phosphate and potassium dihydrogen phosphate, a new ferro-electric type of crystal, are also discussed in detail.A chapter is devoted to information based on new measurements on a number of piezo-electric crystals belonging to different symmetry classes. The considerable theoretical progress made in the past few years in analysing the properties of ferro-electric types of crystals and in locating the causes of the ferro-electric anomalies in Kochelle salt, potassium dihydrogen phosphate and barium titanate is clearly presented. Dr. Mason also discusses the electro-strictive effect in Rochelle salt and barium titanate, and presents data on the behaviour of these new ceramics as electro-acoustic transducer elements. Applications of ultrasonics for the measurements of the physical properties of gases, liquids and solids are given in the remaining part of the book, which contains much new information. The application of tensor calculus to the equations of liquids, gases and solids is included in an appendix. This book is not written as a text-book but as a report of the comprehensive work carried out in the Bell Telephone Laboratories during the past few years. It should appeal not only to the crystal technologist and physicist but also to the crystal -lographer and chemist. Dr. Mason states that the book can be regarded as an introduction to the study of piezo-electricity; he is a very modest man.