Publisher Summary When dirt particles in lubricating oils are considered, it is obvious that the cleaner the oil the better it is. Even dirt particles much smaller than the mean film thickness cause wear if they are hard. If they are large enough to penetrate the oil film thickness, they cause local stresses at the surface and thereby shorten the life of the bearing considerably. The reduction of bearing life with water concentration is steepest when the water can be dissolved in the oil, but even at high concentrations, more water gives shorter life. Stresses below asperities stresses can now be modelled theoretically and, with the use of the new life theory, predictions of life reduction in the presence of contaminants can be provided. This means that it always pays to keep the dirt and water concentration in the lubricant as low as possible. The chapter presents a 3-D mapping of surface areas in which dents are generated by debris.