The present invention relates to a sensor arrangement. In particular, a sensor for the measurement of magnetically active or ferrous debris, for example, as generated as a result of machine wear. There is provided a sensor (5) having a magnetic sensor head adapted to produce a magnetic field, —a magnetic field sensor (10) responsive to changes in the magnetic field; a temperature probe (15); and processing means (50) for determining the amount of debris accumulated on the magnetic sensor head based on both magnetic field and temperature data.