Abstract: An apparatus for monitoring particulate matter in the fluid comprises a filter means (28) through which fluid is arranged to flow. A pressure sensing means (46) senses the fluid pressure difference across the filter means and produces a pressure signal indicative of the magnitude of the pressure difference. A processing means (49) receives the pressure signal and evaluates therefrom the rate of change of the magnitude of the pressure difference in order to give an indication of particulate matter levels in the fluid. In this way on line monitoring of the debris generation in, for example an oil system, can be effected so that abnormal increases in debris can be detected and long term trends in the build-up of debris can be observed. A particular form of filter medium is also disclosed wherein the filter mean can be transported so that a filter record can be maintained and systematic X-ray analysis of the filter record can be effected.