The use of an optical fiber as a distributed sensor for detecting, locating, and (with suitable signal processing) classifying intruders is proposed. Phase changes resulting from either the pressure of the intruder on the ground immediately above the buried fiber or from seismic disturbances in the vicinity are sensed by a phase-sensitive optical time-domain reflectometer (Φ-OTDR). Light pulses from a cw laser with a narrow (kHz range) instantaneous linewidth and low (MHz/min range) frequency drift are injected into one end of the single mode fiber, and the backscattered light is monitored with a photodetector. Results of analyses and experimental studies to establish the feasibility of the concept are described. Simulations predict a range of 10 km with 35 m range resolution and 30 km with 90 m range resolution. Experiments indicate adequate (several π-rad) phase changes are produced by intruders on foot for burial depths in the 20 - 40 cm range in sand and in clay soils. A phase perturbation in a fiber has been detected and located in a laboratory demonstration of the Phi-OTDR using an Er:fiber laser as the light source. This technology could in a cost-effective manner provide enhanced perimeter security for nuclear power plants, electrical power distribution centers, storage facilities for fuel and volatile chemicals, communication hubs, airports, government offices, military bases, embassies, and national borders.