The wear of bearings and gears in aeroengine is the main cause of engine failures. The debris generated by wear contain important information about the wear status of rotating parts, online sensing of wear debris in lubricant is an effective method to diagnose potential failures of rotating components. Firstly, the mechanism and characterization of wear debris are analyzed, the relationship between wear characteristics and wear particle characteristics and the typical characteristics of debris are described. Secondly, several online sensing technologies of wear debris in lubricant are introduced, including optical method, electromagnetic method, acoustic method and energy method. The monitoring mechanism, technical characteristics, research progress and limitations of the online sensing methods are investigated. Finally, the development trends and challenges of online sensing for lubricating wear debris are systematically discussed.