Friction modifiers have been around for many years. Originally, the application was for limited slip gear oils, automatic transmission fluids and multipurpose tractor fluids.Since fuel economy became an international issue, initially to reduce crude oil consumption, friction modifiers have been introduced into automotive crankcase lubricants as well. The current emphasis is to improve the fuel efficiency through the engine lubricant and to reduce emissions to the environment.This paper describes the chemistry of the various organic friction modifiers as well as the non-organic types. A basic understanding is given on the mechanisms how and why these products work as friction modifiers and what is known about structure - activity relationships.Definitions are given about the various lubrication regimes as well as figures to what extend these regimes are present in current engine tests measuring fuel economy. In addition, it is described which type of friction modifiers is most active in the various lubrication regimes.Factors influencing the friction reducing properties, like base fluid characteristics and competing additives, are covered and appropriate tests to measure frictional properties are described. Finally, some recent work regarding the effect of some friction modifiers on frictional levels, determined with a pin-on-ring tribometer, is shown.