Many of the lectures printed in this volume have formed the basis of a series given at Teachers College, Columbia University, during the summer sessions of 1914 and 1915, and during the academic year 1914-1915. Others were addressed to parents, to groups of men, to women's clubs, and to conferences on sex-education. In order to avoid extensive repetition, there has been some combination and rearrangement of lectures that originally were addressed to groups of people with widely different outlooks on the sexual problems. Several years ago the late Dr. Prince A. Morrow announced that a volume dealing with many of the timely topics of sex-education was to be prepared by the undersigned with the advice and criticism of a committee of the American Federation for Sex-Hygiene; but even before Dr. Morrow's death it became evident that this plan was impracticable. Three members (Morrow, Balliet, Bigelow) of the original committee collaborated in a report presented at the XV International Congress on Hygiene and Demography. Since that time the writer, working independently, has found it desirable to reorganize completely the original outline announced by Dr. Morrow. In accordance with a declaration made voluntarily in a conversation with Dr. Morrow, the author considers himself pledged to devote all royalties from this book to the movement for sex-education. Among the many persons to whom is due acknowledgment of helpfulness in the preparation of this book, the author is especially indebted for suggestions to the late Dr. Prince A. Morrow, to Dr. William F. Snow, Secretary of the American Social Hygiene Association, and to Dr. Edward L. Keyes, Jr., President of the Society of Sanitary and Moral Prophylaxis; for constructive criticism, to his colleagues, Professor Jean Broadhurst and Miss Caroline E. Stackpole, of Teachers College, who have read carefully both the original lectures and the completed manuscript; and to Olive Crosby Whitin (Mrs. Frederick H. Whitin), executive secretary of the Society of Sanitary and Moral Prophylaxis, who has suggested and criticized helpfully both as a reader of the manuscript and as an auditor of many of the lectures delivered at Teachers College.