Coolant filters have been used for over 30 years by heavy duty engine builders but little has been published in the technical literature documenting their performance. In heavy duty cooling systems a supplemental additive package is periodically added to the system (usually at the oil drain) to prevent the coolant from becoming corrosive and to stop the build-up of deposits which cut down on heat transfer. Not only is the coolant filter the most convenient and reliable method to deliver the supplemental additive to the cooling system, it removes debris from the coolant which can cause deposits and wear, aggrevate corrosion, and even plug heat exchangers. Additionally, the used coolant filter serves as a diagnostic trouble shooting tool. The results of extensive lab and field evaluations are reported documenting the benefits of coolant filtration.