The two years which have elapsed since the initial discussion of the outlines of the book have thoroughly convinced us of the need of such a work. The Engineers, the Medical Department, and most of the other branches of the Army have their recognized textbooks and manuals. There has been no way, however, by which the uninformed can check the accuracy of statements regarding Chemical Warfare. The present volume will serve, in a measure, to fill this gap. That it does not do so more completely is due in part to the fact that secrecy must still be maintained about some of the facts and some of the new discoveries which are the property of the Service. Those familiar with the work of the Chemical Warfare Service will discover, though, that the following pages contain many statements which were zealously guarded secrets two years ago. This enlarged program of publicity on the part of the Chief of the Service is being justified every day by the ever increasing interest in this branch of warfare. Where five men were discussing Chemical Warfare two years ago, fifty men are talking about the work and the possibilities of the Service today. It is hoped that the facts here presented may further increase the interest in Chemical Warfare, for there is no question but that it must be recognized as a permanent and a very vital branch of the Army of every country. Reasons for this will be found scattered through the pages of this book. It should be explained that this is in no sense a complete historical sketch of the development and personnel of the Chemical Warfare Service. At least two more volumes are needed,—one on the Manufacture of Poisonous Gases and one on the Tactics of Chemical Warfare. We have purposely refrained from an attempt to give credit to individuals for the accomplishments of the various Divisions of the Service, because such an attempt would have made the book too voluminous, and would have defeated the primary purpose, namely, that it should present the information in as concise manner as possible. The published and unpublished materials of the files of all the Divisions have been freely drawn upon in writing the various chapters, and many old C. W. S. men will undoubtedly recognize whole sentences which they wrote under the stress of the laboratory or plant “battle front.” May these few lines be an acknowledgment of their contributions. Those who desire to consult the literature of Chemical Warfare will find a fairly complete bibliography (to about the middle of 1919) in “Special Libraries” for November, 1919. Special acknowledgment is made to Dr. G. J. Esselen, Jr., for having read the manuscript and for helpful and constructive criticisms. Many of the figures are reproduced by permission of the Journal of Industrial and Engineering Chemistry; those showing the Nelson cell were furnished by the Samuel M. Green Company.