A system of monitoring operation for a conveyor belt installation includes a conveyor belt having steel or other relatively magnetically permeable reinforcing material, the system including, in proximity to the conveyor belt, a magnetic field generator, and a sensor unit at a position downstream from the field generator, the sensor unit sensing the magnetic field emanating from the passing conveyor belt, and a monitoring unit to receive data related to the magnetic field properties sensed by the sensor unit during a plurality of passages of each part of the length of the conveyor belt past the sensor unit, the monitoring unit incorporating comparison elements to compare a received set of data with an earlier received set of data, and output elements to provide an output signal representative of reinforcement damage or deterioration when subsequently received data has departed from earlier received data by more than a prescribed extent.