The world-wide operations of the U.S. Army in World War II involved an enormous amount of construction and the performance on a comparable scale of many other missions by the Corps of Engineers. This is the first of four volumes that will describe the participation of the Engineers in the war and the contribution they made toward winning it. Better known to the public in peacetime for its civil works, the Corps by the time of Pearl Harbor had turned almost its full attention to military duties. At home the Engineers took over all military construction, and prepared hundreds of thousands of Engineer troops for a variety of tasks overseas. These tasks included not only construction but also a number of other duties more or less related to engineering both in rear areas and in the midst of battle. In performing these duties in World War II the Army Engineers gained a proud record in combat as well as in service. This first volume tells how the Corps organized and planned and prepared for its tasks, and in particular how it trained its troops and obtained its equipment. The volumes still to be published will describe the huge program of military construction in the United States, and Engineer operations overseas in the European and Pacific areas. One of the objectives of the technical service volumes of the Army's World War II series is to capture the point of view of the service concerned. In doing so the authors of the present history, by thorough research and diligent solicitation of assistance, have also brought to their story a broad perspective, and they have told it with a felicity that should make their work a valuable guide to the Army as a whole, to the thoughtful citizen, and to the Engineers who served and who continue to serve the nation in war and in peace.