Lube oil contamination is a primary reason for engine wear. The wear processes promoted by oil contamination lead to diminished fuel efficiency, shorter useful oil service life, reduced component life, and loss of engine performance. Contaminant particles responsible for this damage are in the size range of the dynamic lubricant films separating moving engine component surfaces: 10 microns and smaller. By making simultaneous contact with opposing surfaces these harmful particles focus the load onto a small area, degrading the surface. This paper reviews the nature of lubricant contamination, the modes of lubrication, and the primary mechanism of engine wear. The correlation between contamination and war of engine components, as well as several important studies quantifying the effect of contamination on engine wear and performance, are discussed. It is concluded that operating with clean lube oil, maintained by 10 micron filtration (10>75), results in a diesel engine having better fuel economy, longer oil change intervals, superior performance, and greater durability.