Compressors are vitally important in many industrial technologies. For example, compressors are used in nearly every industry, including steel, automotive, petroleum, mining, food, gas production, and storage and energy conversion. The purpose of the compressor lubricant is to reduce friction and wear of the working parts of a compressor, such as bearings, gears, and pistons; reduce internal leakage; and if the oil is compressed in the compression zone, to provide heat transfer to reduce the temperature of the gas being compressed. In addition, a properly formulated compressor lubricant should provide corrosion protection and be sufficiently stable to minimize the potential for deposit formation on hot surfaces within the system. The following topics will be discussed in this chapter: a basic tutorial on gas compression, the classification, operation, and lubrication of typical gas compressors, lubricant types and classifications, solubility of common gases, and a review of recommended compressor lubricant testing.