It has been recognised for many years that the majority of failures in fluid power transmission components can be directly attributed to contamination. Furthermore, it has been observed that the size and concentration of these contaminant particles effects the wear rate of components. As a consequence of the reduction in acceptable dimensional tolerances of electro/hydraulic actuators, the acceptable cleanliness levels of fluid power oils have also reduced.Full-flow barrier media filtration alone can no longer cope with the necessary cleanliness levels for the smooth and consistent operation of electro/hydraulic actuators over their desired service life. By-pass filtration is the key to controlling oil cleanliness between services. Unfortunately, traditional barrier media filters of a suitably low micron rating tend to be excessively large to meet the desired service interval. However, centrifugal by-pass filtration offers a solution to controlling the oil cleanliness level without compromising filter size.This paper briefly explains the principles of traditional barrier media and centrifugal by-pass filtration and highlights some of the advantages that self-powered centrifugal oil cleaners have over barrier media filters such as the physical size difference over similar service life. By-pass centrifugal oil cleaners are shown to offer excellent cleanliness control when operated under optimal conditions. It is these optimal conditions which are addressed in detail by this paper. The paper concludes by presenting some laboratory test results showing how contamination control can be achieved using a self-powered centrifugal oil cleaner in by-pass. It is demonstrated through laboratory testing, that through careful rotor design and control of operating conditions, significant improvements to fluid cleanliness levels can be maintained.