The two categories of outdoor electronic protection are fence-hung and free-standing. Hanging systems include vibration contacts, electrical or magnetic fields, and low frequency audio scanners. Free-standing systems are beam-breakage microwave scanners, stacked or single photoelectric beams, electro-magnetic volume protection, buried seismic detection, and video motion detection. Choosing a system depends on the degree of security needed, the direction of possible attack, and the character of the location. Both users and installers should try to minimize the occurrence of nuisance alarms, and users should try to keep conditions relatively static. Ideally, the security should be planned before the fence is constructed, because this will influence the type of system that is chosen. For example, a microwave link might not be applicable if there is not enough space between the fence and valuable items. Fences with general purpose barbed tape provide both psychological and physical deterrents. Seismic systems detect underground vibrations, particularly in quiet areas. The E-field system detects the presence of a human being within an established electrostatic field in high risk facilities such as correctional facilities or nuclear institutions. Other systems include fencing material that creates a high-frequency mechanical shock wave when damaged, video motion detectors, and coaxial cables that act as microphones for tools of intrusion. Photographs are included.