The monograph examines the historical experience of the development and combat use of forces and means of naval electronic intelligence (communications intelligence) in the first half of the 20th century - the period of two world wars and many smaller military conflicts. Research of previously closed documents and a wide range of other sources and literature , taking into account the evolution of military affairs and radio reconnaissance in Russia and abroad, allowed the author to identify trends, changes in the list and content of tasks solved by the domestic maritime DER, to establish the main directions for the development of methods and methods of its conduct; to determine the place of naval DER in the reconnaissance support of the fleet and its role in solving the operational and strategic tasks facing them by domestic fleets and groupings of forces. The experience of the Russo-Japanese and First World Wars, the Civil War and military intervention in Russia, armed conflicts near the island is taken into account. Hasan and in the area of the river. Khalkhin Gol, Soviet-Finnish, Great Patriotic and Soviet-Japanese wars. The history of foreign maritime DER (Great Britain, Germany and the USA) is presented by an analysis of its place and role during the First and Second World Wars, as well as the Korean War. In preparing the work, materials from the funds of federal and departmental archives were used, most of which were introduced into scientific circulation first. The appendices systematize data on the activities of the forces and assets of the domestic naval DER and on the development of these assets, as well as data on the impact of the results of their combat use on the effectiveness of reconnaissance and the solution of operational tasks. Without pretending to be exhaustive and absolute objectivity, the author puts forward scientific assumptions and gives assessments , draws conclusions that are of practical significance for specialists in the field of military intelligence and military history today. This book is also of interest to ordinary readers who are keen on studying the history of wars, the navy and military intelligence.