A method of monitoring the wear of relatively moving parts, particularly inaccessible moving parts of internal combustion engines, comprises fixing a length of radioactive material into the part to be monitored, so that one end of the material is flush with the wearing surface, and measuring the loss of radioactivity as the surface of the part and radioactive material are worn away.An internal combustion engine having a length of radioactive material fixed into a relatively moving part, as described above, and a radiation detector which may be fitted to the outside of the engine is also claimed.The method may also be used to measure wear in, for example, pumps or compressors.