In any hydraulic system, we can, without reservation, say that the cleaner the fluid the better. But, since absolute cleanliness is usually unobtainable, it becomes important that we can readily measure and define the degree of cleanliness in a practical way.Two parameters seem important: number of particles and particle size. In examining hydraulic components as to type and usage, two areas of particle size are of special significance, 5 and 15 micrometres (μm). By counting all particles in a fluid sample above 5 μm and also counting all particles above 15 μm, two indexes are given from which a contaminant code* is derived. The thesis of this paper is that this code is useful in predicting the performance of specific components with the fluid.