It is a medical book. Dr. Moll is a gifted physician of long experience whose work with those problems of medicine and hygiene which demand scientific acquaintance with human nature has made him well known to experts in these fields. In this book he has undertaken to describe the origin and development, in childhood and youth, of the acts and feelings due to sex; to explain the forces by which sex-responses are directed and misdirected; and to judge the wisdom of existing and proposed methods of preventing the degradation of a child's sexual life. This difficult task is carried out, as it should be, with dignity and frankness. In spite of the best intentions, a scientific book on sex-psychology is likely to appear, at least in spots, to gratify a low curiosity; but in Dr. Moll's book there is no such taint. Popular books on sex-hygiene, on the other hand, are likely to suffer from a pardonable but harmful delicacy whereby the facts of anatomy, physiology, and psychology which are necessary to make their principles comprehensible and useful, are omitted, veiled, or even distorted. Dr. Moll honors his readers by a frankness which may seem brutal to some of them. It is necessary.