First published in 1952, this volume is a history of the evolution of an air defense system in the United States from approximately the end of World War II through June 1951. The decision to include in the present work an account of the pioneer air defense efforts of its predecessors as well as major developments within the Air Defense Command for the first half of 1951 was prompted by certain important historiographical considerations. Foremost of these was the necessity for placing the present Air Defense Command's air defense effort in its proper historical setting. Unfortunately, there was a year and one half period (June 1947 -December 1948) when no history of air defense was written. And while Continental Air Command histories for the period January 1949-June 1950 contained much information of value, the diversity of missions of that command had the effect of obscuring its air defense contributions as revealed in its official history. It is frankly admitted that a great wealth of data on certain of the subjects treated herein remains to be exploited. It is also acknowledged that several subjects deserving of treatment were not presented either for lack of time or because of a paucity of reference data. It is felt, however, that the considerations necessary to an historical appreciation of the complex subject of air defense have been revealed in the present work. In future semi- annual histories and historical monographs any omissions appearing in this volume will be corrected. The greatest of care was exercised to substantiate all facts and analyses in the history with pertinent documentation. Also, the officers of the headquarters were solicited to read and comment freely on those chapters the subject matter of which was most familiar to them. If, in spite of these precautions, inaccuracies of fact or misleading interpretations are subsequently revealed, immediate correction will be made in all outstanding copies.