The NLGI definition for a lubricating grease is as follows: "A solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included." The properties of the grease are generated through the combination of the base oil and thickener along with the manufacturing process, but for the majority of applications, these need to be modified with performance additives. Additives for lubricating grease are split into two classes: physical and chemical. Physical additives are chemically inert and undergo no chemical reaction. They affect the physical properties of the grease: structure and rheology; oil bleeding; and water tolerance. Chemical additives, as the name suggests, are chemically active. There are additives that react at the surface of the lubricated contact such as corrosion inhibitors, antiwear and extreme pressure (EP) additives and those that react within the matrix of the grease such as antioxidants and dropping point enhancers. Many additives used in grease are similar to those used in liquid lubricants, but many are unique to grease. The treat rates used in grease are also much higher than used in typical industrial lubricants. All these types of additives will be covered along with typical treat rates used in grease.