This chapter was generated to give an overview of current base oil specifications and methods of analysis. Base oils are components of many finished products, including lubricating oils, automotive and diesel engine oils, lubricating greases, white mineral oils, rubber processing oils, industrial oils, and refrigeration and heat transfer fluids, of which most are discussed in detail in other chapters. Group I, II, III, and some Group V base oils are manufactured from various refining processes, including both crude oil refining and used oil re-refining. They are commonly referred to as base oils, base stocks, mineral oil-based fluids, mineral oils, lubricant base fluids and hydrocarbon base oils throughout the industry. Oil properties vary based on the feedstock, refining process, and manufacturing parameters. Typically, lubricant base oils distill in the range of 280°C to 560°C. Group II and Group III base oils are low in aromatics and high in isoparaffinic content, which yield desirable properties for lubricant applications. This chapter represents the first step in describing properties of lubricant base fluids affecting product performance. Test methods have been identified to characterize the composition and performance of base oils in addition to verifying their consistency.