Oil monitoring constitutes an important and essential component of condition monitoring technologies and has distinguished advantages in revealing wear,lubrication and friction conditions of tribo-pairs.Newly developed on-line/in-line oil monitoring technologies extend the merits into real-time applications and demonstrate significant benefits in maintenance and management of equipment.This paper comprehensively reviews the progress of on-line/in-line oil monitoring techniques including sensor technologies,their scopes and industrial applications.Based on the existing developments and applications of the sensors for oil quality and wear debris measurements,the trends for future sensor developments are discussed with focuses on accurate,integrated and intelligent features along with exploring a fundamental issue,that is,acquiring the knowledge on degradation mechanisms which has not received sufficient attention until now.Current status of applications of on-line oil monitoring is also reviewed.Although limited reports have been found on this topic,increasing awareness and encouraging progress in on-line monitoring techniques are recognized in applications such as aircraft,shipping,railway,mining,etc.Key fundamental issues for further extending the on-line oil monitoring techniques in industries are proposed and they include long-term reliability of sensors in harsh conditions,and agreement with fault or maintenance determination. Oil monitoring constituted an important and essential component of condition monitoring technologies and has distinguished advantages in revealing wear, lubrication and friction conditions of tribo-pairs. Newly developed on-line / in-line oil monitoring technologies extend the merits into real-time applications and demonstrate significant benefits in maintenance and management of equipment.This paper comprehensively reviews the progress of on-line / in-line oil monitoring techniques including sensor technologies, their scopes and industrial applications. Based on the existing developments and applications of the sensors for oil quality and wear debris measurements, the trends for future sensor developments are discussed with accurate on integrated, intelligent features along with exploring a fundamental issue, that is, acquiring the knowledge on degradation mechanisms which has not received qualified until now. Current status of applications of on-line oil monitoring is also reviewed.Although limited reports have been found on this topic, increasing awareness and encouraging progress in on-line monitoring techniques are recognized in applications such as aircraft, shipping, railway, mining, etc.Key fundamental issues for further extending the on-line oil monitoring techniques in industries are proposed and they include long-term reliability of sensors in harsh conditions, and agreement with fault or maintenance determination.