This book traces the development of American radio intelligence at the operational and tactical levels from its beginnings in World War I through the end of World War II. It shows that signals intelligence is useful to the tactical and operational level commander. The study recommends the Army rethink signal intelligence support to the various echelons, primarily through changes to tables of organization and equipment. The book covers the initial appearance of radio intelligence units on the battlefields of France in the first world war, identifying specific instances where radio intelligence played a role in a command decision. It also looks at training and doctrine in the period between the two world wars. The book also covers the organization, doctrine, and training of radio intelligence units as they prepared for combat It provides a glimpse into the intelligence support provided to the corps, army, and army group commanders during World War II through examination off actual intercept operations. Where possible, the study compares and contrasts German radio intelligence units and operations with their American counterparts.