Hydraulic machinery systems are extensively used in civil, military, and marine industries. Mechanical friction and the associated wear are common in the moving components of the hydraulic system, especially for the high-speed rotary and reciprocating machinery. The moving parts of the hydraulic machinery generate debris particles, due to wear and tear. The presence of debris in the hydraulic or lubrication oil is a key indicator of growing damage in the system. If the debris can be detected at the right time, the damage can be identified and a future failure of the system can be avoided. Thus, to enable on-line condition monitoring of such systems, highly sensitive on-line debris detection systems are required. This thesis presents the details of the research conducted to develop (i) a highly sensitive debris sensor for hydraulic oil reservoirs, and (ii) an in-line debris sensor for high-pressure flow tubes. The designs presented overcome several important limitations or demerits of the existing wear debris sensors. Suitable prototype sensor units were designed and developed for the proposed schemes and tested on a hydraulic test bench. To optimize the sensor, appropriate simulation studies were carried out before the prototype development. The prototype sensors were tested extensively using different known sizes of debris and validated the performance. The main challenges addressed in this work, proposed solutions and the conclusions made based on the test results will be presented in the talk.