WHOEVER takes up this book with the idea that the aged can be transformed into sprightly adolescents will be disappointed. A work based entirely on evidence of a scientific nature, as is the present volume, cannot have such an end in view, since it is altogether unattainable—at least with what knowledge is now available. But while it is still impossible for us to create a young man out of an old one, it is quite within the bounds of possibility, as we shall endeavor to demonstrate herein, to prolong our term of youthfulness by ten or twenty years. In other words we need no longer grow old at forty or fifty; we may live to the age of ninety or one hundred years, instead of dying at sixty or seventy. All this can be brought about by the observance of certain hygienic measures, and by improving the functions of a certain few of the glandular structures in our body, provided incurable organic disorders have not already too gravely compromised one or more of our main organs. In a communication to the Paris Biological Society, presented in our name by Dr. Gley, Professor of Physiology at the University of Paris, and in an address delivered before the Brussels Royal Society of Medical and Natural Sciences, we described old age as a chronic disease due to degeneration of the glands with internal secretions (hereinafter frequently referred to as the ductless glands), of the thyroid, the sexual glands, and the adrenals in particular. In this work we will show that this degeneration is amenable to treatment, just as are chronic diseases in general. The facts herein presented are illustrated and sustained by numerous experimental and clinical observations. Being desirous of proving the correctness of all our statements, we have had to enter, sometimes very fully, into the question of the ductless glands, in order to point out the marvelous influence they exert upon the various vital functions. In view of the fact that the ductless glands have already been treated in a very elaborate and exhaustive manner by a well-known American author, Professor C. E. de M. Sajous, of Philadelphia, in his work on the “Internal Secretions” (2 volumes) which introduces many new thoughts and important discoveries, we have paid particular attention to the thyroid and sexual glands, which we have carefully studied anatomically, histologically, experimentally and clinically.