Physical security measures need to be utilized as part of a comprehensive counterespionage program. Physical security will aid in the protection of both life and sensitive information. The physical security sensors described in this chapter include both current and older technology. Older security sensors and systems are discussed because they may still be found at some locations and they can still be utilized as part of a total security system. If there are upgrades to the system and/ or new construction, the most current technology security sensors and system should be utilized.