Condition based monitoring has become essential in maintaining and extending the health of high-speed rotating and reciprocating machinery. One effective approach to detect signs of potential failure of a rotating or reciprocating machine is to examine the wear debris in its lubricating oil. Recent patents and research articles on on-line wear debris detection in lubricating oil are reviewed in this paper, including electrical impedance detection, acoustic detection, optical detection, pressure detection, online x-ray spectrography, resonant frequency detection, and electrostatic charge detection. Advantages and limitations of each detection method are discussed.