This book is a concise primer that teaches the principles, strategy, and tradecraft of counterintelligence (CI). CI is often misunderstood and narrowly equated with security and catching spies, which are only part of the picture. As William R. Johnson explains, CI is the art of actively protecting secrets but also aggressively thwarting, penetrating, and deceiving hostile intelligence organizations to neutralize or even manipulate their operations. Although written during the late Cold War, this book continues to be valuable for intelligence professionals, scholars, and students because the principles of CI are timeless and the book captures the wisdom of one of the most accomplished counterintelligence officers in the CIA's history. CI remains a critical component of today's intelligence missions, even when the opposition is a non-state group such as al-Qaeda. The book will also interest general readers because its lively narrative and detailed descriptions of tradecraft reveal the real world of intelligence and espionage. Many intelligence professionals consider this book a classic, and the CIA's suggested reading list calls it "the best introductory book on the subject."