Oil analysis programs are used to monitor the condition of diesel engines. From the standpoint of engine protection, the size and concentration of oil contaminants are as important as their chemical composition. Wear rates are related to the contaminant size distribution in the oil and degree of filtration. Spectroscopic analysis does not provide particle size information and is incapable of measuring the particularly damaging particles larger than 10 μm. Particle size analysis, when used to complement the spectroscopic analysis, provides a better description of the condition of lubricating and transmission oil. Particle size data can be obtained by particle counters, though in the past it was difficult to obtain reliable data. Recent developments overcome many of these difficulties and permit reliable interpretations of particle size data. This allows better decisions to be made concerning the engine and oil conditions and the filtration requirements of the system.