ESDU 83030 enables an appropriate filter to be selected for a lubricant circulation system by estimating the required filter rating or filter stopping power. After describing the features of a typical lubricant circulation system, the various purposes for which filters are incorporated are explained. Filter performance is discussed in terms of the probability of its capturing a percentage of particles of a certain size, and the concept of filter rating and beta ratio explained. The selection procedure, to give a full flow filter that will prevent contaminant particles from damaging components of a machine, is then developed. It establishes a suitable filter rating taking account of expected lubricant film thickness and the surface roughnesses of the opposing surfaces. Worked examples illustrate the use of the procedure and a section discusses practical experience with hydrodynamically-lubricated thrust and journal bearings, rolling bearings, gears, seals and couplings.