As crime and terrorism continue to grow, the need for alarm systems increases. Both criminal and terrorist acts depend on surprise, and an effective alarm system, therefore, should give an early warning of acts in progress and possibly detect tampering or attempted entry. Purely destructive acts (sabotage or assassination) are most difficult to protect against. Alarm systems consist of sensors that monitor the environment, triggering circuits that communicate sensor changes, indicators that translate sensor changes into signals identifiable by humans, a communications link between these components, and a power source. The primary purpose of an alarm system is detection; protection is provided through the combination of the alarm system with a response mechanism. A variety of sensors are available, including point or spot devices, such as switches, pressure sensors, foil or tape, vibration sensors, and wires. Area or volume alarm sensors monitor three-dimensional spaces and large volumes of space. These incude types of sensors such as: ultrasonic, infrared, monostatic microwave, audio, light beam, capacitance proximity, perimeter, bistatic microwave, buried, fence-associated, electric-field or E-field, and closed-circuit television motion detectors. Residential security system considerations include the structuring of communications links through telephones or existing cable television lines for outside response forces. These systems are generally ineffective in detecting terrorist acts due to the length of response time, the nature of communications lines, and the fact that perimeter alarms are most commonly used and are most easily circumvented. Duress alarms which can be manually activated in emergencies should be located in residences vulnerable to terrorist attack. Systems for office environments can provide protection during the day and when the area is unoccupied through the selective use of sensors. Industrial systems should provide perimeter protection as well as indepth protection. The characteristics and installation of a wide variety of sensor types -- including ultrasonic, monostatic microwave, and vibration sensors -- are described and trends in alarm systems are discussed. The characteristics of indicators, power sources, and communications links are also detailed. Illustrations, 23 references, and 25 system criteria references are provided.