Abstract: A monitoring system continuously monitors fibre optic cables for faults and then enables automatic location of the fault in the cable. In the system an optical monitoring signal is generated and connected as an output signal to a first end of the fiber optic cable for transmission to the second end. The signal is then returned through the fiber optic cable to provide a return signal at the first end of the cable. A comparator at the first end compares the output signal and the return signal and provides corresponding difference data. An optical time domain reflectometer detects the location of a signal loss along the fiber optic cable and a data processing system determines the location of the signal loss from the corresponding difference data. A single optical time domain reflectometer can be used for monitoring a plurality of fiber optic cables.