The contamination level of the fluid within a hydraulic system is a reflection of not only the particle removal capability of the filter element, but also the contaminant ingression characteristics of the system. Furthermore, the amount of contaminant permitted to enter the fluid past exclusion devices has a direct influence on the filter requirements-separation efficiency and capacity. Thus, without some means of establishing the effectiveness of contaminant exclusion devices, no realistic analysis can be made regarding the contamination level of a hydraulic system.This paper presents the analytical expressions that can be used to describe rigorously the contamination control of a hydraulic system. The importance of contaminant ingression is dramatically illustrated by realistic test results. The correlation of the theoretical considerations with characteristic field conditions is accomplished using the results of a wiper seal study. The interpretation expounded is unique and has broad implications in the earthmoving industry.