IN THIS CHAPTER WE DISCUSS GEAR LUBRICANT technology. We deal with the topics of gear types, gear metallurgy, and types of gear damage. An understanding of all these subjects is important to formulating a suitable gear lubricant. Performance standards and testing of the gear lubricants, both automotive and industrial, are also described. The chapter concludes with a number of formulation examples. The primary functions of a gear oil are to reduce friction between the gear surfaces in contact, dissipate heat, and provide extreme pressure protection to gears and axles against fatigue, scoring, and wear damage, under boundary lubrication conditions [12]. In automotive applications, the gear lubricant must also minimize shock and the noise resulting from the motion of the meshed gears in the rear axles, thereby providing a smooth and quiet operation during acceleration.