This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book examines intelligence analysis, not covert action, counterintelligence, propaganda, disinformation, or other such topics. It discusses the detection and the frustration of the activities of foreign agents, counterintelligence has little bearing on the problems. The book devotes little attention to the vast area of disinformation, the frequently successful effort to affect processes of judgment between countries and statesmen and to distort public opinion through disguised means. The human intelligence, which had never been quite as important as commonly believed, further declined in perceived importance, and the fields wherein technical intelligence could achieve results were strictly limited. The book concludes that, far from being an invisible government, far even from wielding great influence in the councils of state, intelligence has frequently been disregarded or ignored by decision makers.