This paper reviews piston and ring friction which can account for 65% of the mechanical friction in an internal combustion engine. It shows that cylinder liner lubrication is predominantly hydrodynamic with localized contact between ring and liner at TDC firing. The degree of contact may increase during transient conditions. Piston ring friction in the hydrodynamic region is proportional to the square root of the viscosity. The viscosity is affected by temperature and pressure which can reach peak values of 340°F and 4000 psi, respectively. Gains in fuel economy through viscosity reductions have been reduced in the last 25 years due to changes in piston and ring design.