Abstract: An intruder detection system wherein light pulses are fed from a light source to a detector via a transmitting optical fibre, an optical terminator and a receiving optical fibre, both fibres being either stepped index fibres or poor quality graded index fibres. The fibres are disposed in intimate contact throughout their length within a cable, and compression of the cable at any region of its length sufficient to cause microbending permits light pulses to breakthrough from one fibre to the other. The time interval between arrival at the detector of a light pulse received from the source after passage through the total length of the transmitting fibre and the receiving fibre, and arrival of a breakthrough pulse received after passage through the fibres only so far as the region of microbending and back again, is indicative of the location of the compresion. The cable may be laid around the perimeter of a site to be guarded so that compression will occur when an intruder crosses the cable.